This is pretty much a crit gp guide for dominion players. This is currently my own personal style of gp. I've done pretty well with this build so far. Until I see better dominion players, I will adjust/tweak this build in the future

Pros:
Crit Crit Crit
Parrrley is certainly one of the best skills imo
Squishing all the squishies
Ult can hit anywhere on the map, really big radius AoE attack
Con:
Gp is pretty squishy until you get frozen Mallet
Tend to be focused a lot

Typical 21/0/9 Masteries
Offensive masteries are preferred for gangplank.

I love the extra critical % you get from the marks runes. With the extra defense stats from glyphs and seal, it makes gangplank not as squishy. Armor pen quints are for more damage!

Your most important skill is Parrrley, so you would want to max that out first. Next would be raise morale. The extra attack and movement speed helps a lot when your roaming or trying to get to places. I tend to get remove scurvy at lvl 5 because you want to get out of CCs. Theres really no point maxing that out first because the CD barely goes down and theres always HP pots around the map. GP's Ult is pretty awesome. It allows you to hit anywhere on the map. It also slows other champs. The radius of the Ult is pretty wide, its wide enough to stop people from capping from both sides of the point.

When I have enough time I will explain my mindset of these items.

IMO Ghost works much better than flash in Dominion. With ghost you can use it to either escape or chase.

While in a fight, you want to focus on the squishies. The squishies have the least hp out of everyone else. As Gangplank you can pretty much use one parrrley to take out over 50% of their hp with just one crit. All you have to do next is run up to them and auto attack. If your teammates are in a fight you def have to use E to back them up. E can be used in many ways. You can use E to chase, escape and get to a location much faster. W is pretty much a "get out of CC" card. Whats great about it is that it has a short cd. however, use it wisely cause your pretty vulnerable when its on CD.