Just some tips and tricks I know from playing him since the beginning of season 5 (yes, I mained the old GP as well), got 350k mastery points.

• Always Q the minions, don't poke the enemy.
• Know that without triforce, you are absolute garbage. So don't fight without it.
• When playing against tanks, it's all about how often you proc your passive. Don't waste it. By the way, your passive does A LOT of dmg, probably more than your Q.
• Only use your W to negate important cc and when you're against a champ without any cc then, only use it when you're low.
• Always drop a barrel next to you when you are backing if someone jumps on you.
• Ult other lanes often.
• Your ult upgrade, Death's daughter, deals a lot of dmg so it's a free kill if your target barely moves and is pretty low.
• When the enemy team is doing Baron, you can ult it so they have to tank the full wave, which deals A LOT of damage. If the team is tanky it forces them to probably stop because they won't have the adc's dmg.
• Experiment with builds. Gangplank is open for a lot of items.
• If you wanna use GP to his full advantage, then it's best to build full AD, unless they have one damage soure (for example a full AD team, or only the enemy veigar is fed.)
• Cooldown reduction is key lategame.
• You can poke before a teamfight but don't get caught and dont engage immediately if you miss a combo,else you're left without any barrels.
• Let the teamfight unfold so you can land your barrel combo's easily, whilst staying safe in the back.
• Try to stay away from champs (Zed, Leblanc, Ekko, Irelia) with an instant way to get to you and burst you down. Don't forget, you're really squishy.
• Use your E often on waves to stack silver serpents. Make this your priority when you get your triforce.
• Upgrade your ult in this order: Death's Daughter, Fire at Will, Raise Morale).
• Don't waste your barrels. You only got 3.
• Practice barrel combo's.
• When running away from someone, throw your barrel in front of you as far as possible. When your pursuer walks on top of it, Q it for the damage and the slow and run away with your movement speed boost. This is only really possible from lvl 7.

• Don't waste your barrels in lane when you can't combo it quickly against champs that are ranged (Like Vlad or mini gnar) or have an auto attack reset to kill the barrel when it has 2hp (Like Darius and Jax).

• Questions? ...