Gangplanks passive is like a mini ignite that activates when auto-attacking a minion or a champion. Also your ability "E" resets Gangplanks passive when destroyed.
Use his passive when your enemy is trying to farm, when against range enemy champion this will not work, you will lose the trade.

The "E", if you use your "E" Gangplank will put down a explosive barrel. The Barrel only explodes when you hit it or use "Q" on it... If you use "Q" on "E", DMG SPIKE. The explosive barrel can chain up together and if you destroy one all of them will explode in a chainreaction
"TIP/in top lane" Put explosive barrel in bush wait for enemy to go farm make your barrels chain so your enemy is in the explosive radius. If hit go and use your passive on your enemy for full dmg, keep doing that and you will win.

Gangplanks "Q" gives you bonus gold. with that bonus gold you can uppgrade your ult.
You can choose, More dmg, More bursts from ult, Movment speed bonus to nearby allies.
Expirement here and decide by yourself.

When farming use "Q" on "E" for fast bonus gold. much more efficent then using "q" and missing some minions.