To give you a brief introduction into Gangplanks spells.

Trial by Fire [P]: It will Apply a dot that burns over time and deals True DMG. Scales with bonus AD and gives you movementspeed.

Parrrley [Q]: Fires a point and click bullet onto an enemy. It is single target, can crit, applys on-hit abilitys, scales with total AD, Give you extra Gold if you kill the target and give you Silver Serpents (SS) which can be used to upgrade his Ultimate.

Remove Scurvy [W]: Will heal you and cleanse CC. Scales with AP and missing health.

Powder Keg [E]: Place a Keg which has 3 HP and loses 1 HP whenever you or an enemy hits it or passively 1 HP every 2/1/0,5 HP depending on your level. You can use Q or a basic attack to make your Keg explode to deal AOE DMG, slow and get MS. If you use Parrrley on the Keg and it kills minions you get bonus Gold and SS for every minion kill. You can store Max 3 Kegs. It costs no Mana and Ignore 40% of the enemys Armor. It also refresh your passive.

Cannon Barrage [R]: Is a Global AOE field. It will last for 8 Seconds and deal constantly magical DPS damage to every enemy in the area. You can upgrade Cannon Barrage up to 3 times with 3 different options. More waves for more DMG, additional ture dmg in the middle of your Ult and movementspeed for your allies.

Gangplank himself is a great pick for competetive gameplay. He has a lot of poke, dmg, push and also slow with Iceborn, Powder Keg (E) and Cannon Barrage (R). You can harass your lane oponent with Parley (Q) and zone him with Powder Keg. Getting extra Gold from your Parley is great for some early pots, wards etc. He offers a lot in Team fights and still deals a decent amount of dmg even with the tanky build. Furthermore, his early game damage with his passive and his low CD Parley is huge and his mid game is also cruel for every oponent.

Pros:
[*] Poke/Harass
[*] Wave clear (depending on your Powder Keg skills)
[*] Zoning
[*] Constant slow
[*] Great mix DMG/Tanky
[*] Can cleanse every 8 second in late game
[*] High early game DMG and good base stats

Cons:
[*] vulnerable for ganks
[*] no real escape, just speed with E and Passive
[*] A few hard Match-Ups
[*] Hard do be effective with E
[*] DMG falls a bit off late (therefore you have Utility)

With his AOE Cannon Barrage and Powder Kegs he can act extremly well in Teamfight comps and Zoning comps, or even dissengage or split push comps. He can deff turrets with R, push with passive and Iceborn (both affects turrets), Slow in Teamfights and can be a great frontline/wall for the enemy ADC. After his core build he is already tanky with high DMG against every squashy enemy (Mid or ADC). He falls a bit off with his DMG in late but then he is a big DPS wall who initiate fights with good Ults and Powder Kegs.

Farming is easy with E and Q. Watch your mana in the early game if you harass or farm hard with your Q. After Iceborn you have enough to poke and Zone the enemy laner to get free farm. Use ur ult for assists and Kills if your team can't get the kill, or deff and farm other lanes. (don't be a **** and steal with your ult, a raging teammate is not worth it). Get your Upgrades with SS to get empowered Cannons and use your bonus Parley Gold for same early advantages.

Gangplank performs well at all. With usual DMG and crit Item builds you may offer some crazy barrel DMG, but you can't survive an ingage or can survive a gank by the enemy jungler/mid laner. With constant slow, great harass and tankyness he can carry his team with Utility and pressure.

Hope this guide could help you.
Cya guys.

KronanT