You use the word "focus" very lightly.
At high elo the team doesn't focus the ad carry,but give it priority over the other champions. The top will get more gold through farming (That much is obvious),but you will have someone else on top instead of you(Be it a riven,nidalee,teemo,olaf w/e).
Also,it's way easier for your ad carry to kill the opponents team with such a reduction of armor.

This guide is neither about lee sin jungle nor shyvana jungle.

Only because you shred their amor they will not focus you at all, that is the problem. Even if some of your numbers are wrong, of course you shouldnot underestimate E with Black Cleaver. But Garen is capable of yo much more then just spinning and do not getting focussed. In the high ranks of elo the players know to not focus the tank, even if he shreds teh armor, because even he does so, the ADC does the damage and so they focus your ADC. This is the thing you want to avoid, the enemy team should focus you, so you can play off your tankyness against them and your ADC can do his damage unharmed. But you can do this also when you are laning top, and you can do it better, because usually you will get cour BC earlier.

It does not matter what an succesfull gank is, because there are so much better champs who can get a kill where Garen would not even force a summonerspell. Why would you want to make it even more difficult? Sure, not everyone is experienced to garen jungle so you have a little element of surprise. But I am sure, when Garen gets a kill with his gank, a Lee Sin or a Shyvana would have, too. They have both faster clears, have better ganks and are better counterjunglers.

You're stil ignorant about team fights.
The ad carry role is to deal all the damage he's able to do in a team fight. This makes them stay out of the range while staying and shooting at this safe distance. Assasins and ap carries will most likely prey on your ad carry,but they'll will also be dealing with your top/mid/supp that should be capable to form a barrier between them and your ad carry.
Tanks should disorganize their team,making them grow apart,but garen should focus on getting his E on them.
And you should realise that when the black cleaver procs enough times it will do(Max 25% armor pen debuff+10% masteries+35%last whisper+All the flat armor penetration gained from runes/masteries/items).
This'll leave their armor reduced to zero allowing your ad carry to literally deal true damage.
Now you also seem confused about what a succesfull gank is.
A succesfull gank is when the enemy laner either uses his summoner spell or when he dies. Any lane is allowed to have some cc,but even if they don't:It'll make it harder to gank,but it will still not be impossible.

You can have damagereduction like a boss, it does not care when they not focus you. Garen should fulfill the typical offtankrole, you focus the (ADC or) APC in the teamfight and burst him down quickly. When noone focusses you, you will cursh the carries and they lose, and when they focus you, you absorb so much damage that your team can easily win the fight because all other members of the team do not get harmed and can deal damage. The W of Garen seems to be made for this, that is right. The problem is, that you have to get fed with kills or lasthits, otherwise you will be just a tank with a cute damageoutput. Sure, you can shred armor, but a fulltank-Garen does not the damage he needs to get focussed, and in this case your Tenacity and Tankyness does not matter anymore.

Like I said before, the W is okay, but it does not help you jungle this much and when you are not fed lategame, what should be a lot more difficult when you are jungling, Garen is not this useful in general.

You can gank with every char, that is right, but it is more important which champions have the best chances on a successful gank. And when you want CC on a lane when you are jungling, why do you not turn things around and go yourself on a lane while you get a CC-jungler?

Not every champ has a escape, but almost all toplaners have Flash. Additionally almost every toplaner has a escape or CC or a speedup. So when you are ganking, the only thing you got is the CC from the lane against the opposing escaping tools. Where Garen just can speed up a bit (and maybe exhaust or flash after the target, but they should have got one of these summoners, too), other junglers have damage and CC like Lee Sin.

In my opinion the CC should be in the jungle. I see definately no problem to pick Garen toplane and then ask for a CC-jungler. The other way around does not seem very attractive to me, most of the CC-monsters do not duel very good so they would create a passive toplane, and Garen in the jungle has no advantages over really many other junglers.

I appreciate your creativity, trying something new is fun and maybe the outcome is better then expected, but better then expected is not always enough.

Let me say it like this: If you really want to play Garen in the jungle, you need the +5 boots (when you have no ganking tool then damage you are useless) and a really good team that knows your strenghts. They should all pick a lane with some hard-CC and you need to do a concrete plan about ganking. This is almost impossible in SoloQ and if you play with teammates, there are better alternatives to get an advantage out of your teamcomposition.

Sigh..You simply can't comprehend how great his W is.
His w allows him to tank everything in his path late game and his E will make him good even late game(Because of the recent changes to black cleaver,his E reduces their armor significantly while your W gives you the ability to receive the abuse).

Lets talk about winning games.

Winning in league of legends on high elo is primarily team play based. Individual skill matters,but having a clear idea how to act when a team fight occurs is mandatory.
Garen played properly will not fall late game,because of the new changes to black cleaver and the new jungle items granting him tenacity and a chunk of hp helps him with holding his tankiness.You can literally move however you want as garen,because of your extreme tankiness and reduction of crowd control.
I consider his W one of his best abilities and you should really up your view on it.
Not only does it increase his armor/magic resistance 20% as passive(Even at the first lvl),but activating will also increase the incoming damage by 30% and giving him CC reduction by 30% for a duration of 2/3/4/5/6 seconds.
I'm not sure whether you noticed that at lvl 5 it is 6 seconds long with 20 seconds cooldown(Not counting cd items which would make it about 12 seconds long).

About ganking:

I once mentioned that garen likes to have lanes with a bit cc in them (Ex:Leona,riven etc). You can also get exhaust if you prefer it that ay,but you can gank with garen if you're familiar with the concept of ganking.
Also,you're assuming that every lane has escape abilities,but this is not always the case.

Honestly,you can gank with almost any champion.Let it be because of cc,damage,range or mobility.

You sometimes got me wrong, I am sorry for that, my English is not this good, but I am trying to improve it.

First, i did not wanted to say that Garens Sustain is bad. Of course you survive pretty long with 5 Pots, for example Amumu has no other sustain then these 5 Pots, too, and Amumu is a great jungler. However, Garens W reduces for 2 seconds the damage for 30%. You will never get with this to the amount of damage other junglers like Malphite or Amumu can absorb. I just wanted to point that out.

Garen shines early and midgame, but he falls off lategame, s3 did not changed that this much. If you are lucky, you can silence the opposing APC, deal a decent amount of AoE damage and absorb a lot of damage because they have to focus you. This only happens if you got kills and/or a great farming. A fed Jax or Irelia would completely destroy the enemy team at this point, while Garen does not offer this potential lategame. I hope, you agree with this. Garen has also not much of a CC-ability like CC-support-junglers have.

I play Garen a lot on the toplane and I know his ablities, I just wrote a little bit missunderstandable. Of course E does not grant speed anymore, but if you max E first, you do not max Q first, what means, the speedboost from the Q is really short, so you do not have this much chasing potential. Also you do not have any slow in your Itemcore. Your ganks are: Coming on the lane, run faster for 1 second and then you can be kited and cced all day long. This does not help at all.

"Rubbish" is maybe a little bit over the top, but Garens W is his most weak ability. Sure, the passive bonus on Armor and MR is nice, even the active is nice in teamfights, but the W is nearly useless before midgame, and for teamfights you should have maxed it, but you do not, because the other ablities are better. That is the Problem, when you need it, it is way to short on duration.

I hope I could clear some of the missunderstandings. I love playing Garen solotop and I am statisfied with my playstyle. Let's face it, Garen is an early- and midgame-champion, and I feel like jungling him wasts his strongest time in the game. The joke of playing Garen is that most of your counterpicks are pretty bad lategameplayers (like Teemo or Nidalee) so can just play safe, when they do not destroy you, you "won" the lane, and all other lategameplayers like Irelia or Jax you can delay them if well played.

Garen does not have the tools to be a good jungler, in my opinion he does not even have the tools to be a decent jungler. His ganks are weak, because you have no maxed Q, his jungling clear is on the same level with Malphite, Garen has no real CC, he is not a lategameplayer and he is not a direct 1v1-champ. His lane dominance is pretty awesome, but he looses this advatage in the jungle.

Well,I considered your previous comments to be somewhat logical,but this one is just ridiculous.
His w lasts 2 seconds and is plenty to hold sustain within the jungle along with his 5 hp potions.
Also,garen in s3 has a great team fight presence&currentlate game.
You also mentioned that maxing your "E" increases your speedboost and claiming you've tried it. W at lvl 5 reduces 30% cc and damage so it is certainly not rubish. You can gank if done properly and jungle clearing time is decent.
You don't have the chase potential?....As a garen?
Well,that's it. I'm done arguing,because you're obviously ignorant about garen and skillset. When you can't see the difference between his Q and E then you shouldn't attempt it.

Garen is definitely not like Malphite in the jungle, because Malphite does not need sustain due to his shield. Also the W of Garen is completely rubbish, it might be good in some situations, but it does not help jungling a lot, because the cooldown is long, the duration is short and !!!

Also I want to say something about your mindset as Garen. Garen was, before Darius came, the best killstealer in the world. Normally you want to help your lanes as a jungler, of course it doe not hurt them when you take some kills when they get assists for that, but Garen has a pretty difficult lategame and a bad teamfight presence, when not fed. So you can decide between you being total useless or weaker lanes.

Also your ganks are pretty weak, if you max E, then you have a speedboost for like 1 second, and no other ganking tool. You can not even counterjungle right, because you do not have the chasing potential. Your jungleclear is pretty awkward in the early levels, where Garen is normally strong on the lane. Sorry, ich tested it, Garen kinda sucks in the jungle.

I meant garen is similar in the way of holding sustain in the jungle(Notice how I added "in the jungle").
He doesn't obtain a passive shield,but his W allows him like malphites to ignore the abuse enough to properly do it.
You use 500 Elo a lot,but let me point out that even around 1000~1800 elo these kind of things will occur.
Saying it might work on 500 elo is just a different way of saying "noob".

Garen is not similar to Malphite. Malphite has some nice CC, a passive shield fitting into the jungle, plenty of Armor, more AoE and more teamfight presence.

I said, on 500 Elo you can win plenty of games with Crittlesticks, that makes Crittlesticks not good. I do not want to point on you and say "You are a noob", because firstly I do not like to do that and secondly because I am a little noob myself. Of course a jungle Garen is a surprise, especially when you have a Lee Sin or an other common jungler on the toplane, the whole enemy composition is unprepared. But even this works better with Jax or Malphite plus a common jungler.

It is an advantage for you when the enemies do not know much about your champ, but for that you should use Trundle and not Garen.