Hi I am Pah Ool from the EUW server and I main jungle. Garen isn't a top tier jungler but he is viable and in certain team comps he is even pretty good. You can also trick the enemies if you first pick Garen and they try to counter him top ;)

I think it's kinda obvious why those summoner spells. Ghost or Exhaust instead of Flash is also viable but in most cases Flash is more useful.

I use the regular strategy on Garen: Wraiths or Wolves -> Smiteless buff which is nearest bot lane -> rushing buff near top lane -> top/mid lane gank. At first Garen doesn't seem to be a fast jungler but after 1-2 recalls he's clearing pretty fast. Even if enemies have escapes you might get easy kills by just flash and silence before he manages to flash or use any other escape. It is required to have a good burst on that lane.