Sorcery bigger barrel dmg
Double-Edged Sword more dmg
Natural Talent bigger dmg and heal
Bounty Hunter Gangplank is a late game champ but if you want more dmg in early game you can go for Oppressor
Wanderer in team fight you dont want to be IN team fight
Assassin at top you will be all the time solo
Merciless you dont want to get Meditation because for early game you have mana crystal and essence and in mid-late game you will have more than 50% of max mana
Bandit you will spam your Q a lot to try to kill your enemy and for set up for gank and 10 golds and free dmg to enemy is good
Intelligence Gangplank works with CDR very well and you dont need armor pen. because your barrels ignore 40% of enemy's armor
Thunderlord's Decree when your enemy want to trade, use your prepared barrel Q and AA

Death's Daughter -> Fire At Will -> Raise Morale
Sheen -> Phage -> Trinity Force
B. F. Sword -> Caulfield's Warhammer -> Cloak of Agility -> Essence Reaver
B. F. Sword -> Pickaxe -> Cloak of Agility -> Infinity Edge
Zeal -> Statikk Shiv
Zeal -> Phantom Dancer
on higher levels you can swap Stealth Ward -> Farsight Alteration

In team fights use your Ult to slow, zone or finish running enemy after team fight,
stay in back line use your barrels to poke enemies before your team go in.
Spit push passive vs towers is strong and you exploding barrels reset it.

I think you can pick Gangplank vs everything he is very flexble champion.
If you want more information about Gangplank, counters or just know how to play him look at other people, i just wanted to show you my build.
some informations about me:
name: Angel603
server: EUNE
division: plat 1
I played just 12 games with Gangplank in ranked but with this build i have 10 wins in row.
And i know my english is bad i am not perfect.