The following information as well as some other interesting info such as trivia and patch history can be found on the League of Legends Wikipedia article for Gangplank: http://leagueoflegends.wikia.com/wiki/Gangplank/Ability_Details
Passive: Grog Soaked Blade
(Innate): Gangplank's basic attacks apply a poison debuff for 3 seconds that can stack up to 3 times. Each stack deals (3 + level) magic damage per second and slows the target by 7%.
Grog Soaked Blade is a passive ability that grants Gangplank an on-hit effect, causing his autoattacks and Parrrley casts to slow and deal magic damage over time, ticking every second. This effect can stack up to 3 times.
Grog Soaked Blade's application does not pop spell shields.
Grog Soaked Blade's damage over time damages Black Shield and the slow is negated.
Grog Soaked Blade does not count as a poison for Cassiopeia's Twin Fang.
(Active): Gangplank takes aim and shoots an enemy with his pistol, dealing damage. If it kills it, he gains extra gold and refunds half the mana cost. This ability can critically strike and applies on-hit effects.
Cooldown: 5 seconds
Cost: 50 / 55 / 60 / 65 / 70 mana
Physical Damage: 20 / 45 / 70 / 95 / 120 (+1.0 per attack damage)
Extra Gold: 4 / 5 / 6 / 7 / 8
Parrrley is a targeted ability that deals physical damage to a target enemy unit. If the target is killed by Parrrley, Gangplank gains a flat amount of bonus gold and an amount of mana equal to half of Parrrley's manacost.
Parrrley procs on-hit effects.
Parrrley will not damage Black Shield.
Parrrley pops Spell shields.
You will not receive a stack of Grog-Soaked Blade when Parrrley is blocked by a spell shield.
Parrrley does not proc spell vamp and Rylai's slow similarly with all spells and abilities that proc on-hit effects.
Parrrley can critically strike.
Parrrley resets Gangplank's autoattack timer.
Parrrley will proc and apply the Sheen, Trinity Force, Iceborn Gauntlet and Lich Bane effects.
Units killed by Iceborn Gauntlet's Spellblade when using Parrrley will also grant the bonus gold.
Parrrley procs the splash damage from Tiamat.
Parrrley is affected by items like Thornmail, Warden's Mail and Ninja Tabi.
Parrrley uses melee autoattack item effects and buffs.
Blessing of the Lizard Elder and Frozen Mallet will apply the stronger melee version of the slow on Parrrley.
Every unit killed by Tiamat's splash damage or the proc from Statikk Shiv will grant Parrrley's bonus gold and refund mana.
This makes it possible to refund more mana than the ability costs (returning the full mana used with 2 kills and returning more than the used mana with 3+ kills).
Parrrley will be blocked by Aegis Protection, dodged by Counter Strike, and parried by Riposte.
W: Remove Scurvy
(Active): Gangplank consumes a large quantity of citrus fruit, which removes all crowd control effects on him and heals him.
Cost: 65 mana
Cooldown: 22 / 21 / 20 / 19 / 18 seconds
Heal: 80 / 150 / 220 / 290 / 360 (+1.0 per ability power)
Remove Scurvy is a self-target ability that heals Gangplank and removes any crowd control effects afflicting him at the time of activation.
Remove Scurvy can remove blinds.
Remove Scurvy does not remove Ignite, the damage reduction from Exhaust, or any 'airborne' crowd control effects (pulls, knock-backs and knock-ups).
Remove Scurvy is one of the few abilities that can remove suppression.
The suppression from Urgot's Hyper-Kinetic Position Reverser will be removed, but if the channel completes both champions will still be moved.
The suppression from Malzahar's Nether Grasp and Warwick's Infinite Duress will be removed, however Malzahar's ultimate will continue dealing damage unless you move out of the tether range or the channel is disrupted.
E: Raise Morale
(Passive): Gangplank's Attack Damage and Movement Speed are increased.
(Active): Gangplank fires into the air, disabling his passive boost but inspiring himself to gain Attack Damage and Movement Speed, with nearby allied champions receiving half that amount, for 6 seconds.
Cooldown: 25 seconds
Cost: 60 mana
Area of Effect: 1400 (estimate)
Cost: 50 / 55 / 60 / 65 / 70 mana
Passive Attack Damage Bonus: 8 / 10 / 12 / 14 / 16
Passive Movement Speed Bonus: 3 / 4 / 5 / 6 / 7 %
Active Attack Damage Bonus: 12 / 19 / 26 / 33 / 40
Active Movement Speed Bonus: 8 / 11 / 14 / 17 / 20 %
Raise Morale passively increases Gangplank's attack damage and movement speed. When activated, Raise Morale is a point blank area of effect ability that causes Gangplank to gain bonus attack damage and movement speed, and all nearby allied champions to gain half of his bonuses. However, Raise Morale's passive bonuses are lost for the duration of the active buff.
When the active buff expires, Gangplank regains the passive bonuses of Raise Morale. It is not removed for the duration of the ability's cooldown.
Raise Morale's active is not an aura, and allies who move near Gangplank after the skill is used will not receive the buff. Likewise allies will keep the buff for the full duration once gained even if Gangplank dies or they move away from him.
R: Cannon Barrage
(Active): Gangplank signals his ship to fire upon the target area for 7 seconds. Each cannonball deals damage. Enemies caught in the area of this ability are slowed, lingering for 1.25 seconds after exiting the area.
Cost: 100 mana
Area of Effect: 575 (estimate)
Cannonball Area of Effect: 275
Magic Damage: 75 / 120 / 165 (+0.2 per ability power)
Cooldown: 120 / 115 / 110
Cannon Barrage is a global ground targeted area of effect ability that marks a target area for 7 seconds, slowing enemy units within for the duration. This slow lingers for 1.25 seconds on enemies moving out of the area. Additionally, every 0.3 seconds a cannonball falls in a smaller area of effect within the marked location, dealing magic damage to enemy units at that point.
Cannon Barrage does not proc on-hit effects.
Cannon Barrage procs spell vamp and Rylai's slow similarly with all area of effect spells and abilities with diminished effect.
Cannon Barrage will damage Black Shield and the slow is negated.
Cannon Barrage's slow will not pop spell shields, but its damage will.
Spell shields only have the potential to negate one cannonball, you can still be hit by other cannonballs and there is no effect whatsoever regarding the slow.
Cannon Barrage grants vision of the targeted area to Gangplank's team for the duration.
Hit-Box size plays an important role during Cannon Barrage. Targets with smaller hitboxes (like minions or Teemo) are statistically less likely to get hit from a cannonball and are therefore likely to take less damage. Targets with larger hitboxes (like Cho'Gath ) are likely to get hit with more cannonballs and take more damage.
Cho'Gath with 6 stacks of Feast and 2 other enlarging buffs ( Wild Growth and Storm Shield Storm Shield) will be big enough to get damaged from every single cannonball if he is in the dead center of the ability.