Welcome to my Gangplank build (first build to upload here).

This may not always be the best way to play Gangplank, but I find myself top scoring almost every game.

Items still need to be picked as per the opposition at times. But being Dominion and short games, I often build full damage, as survivability isn't so crucial.

Why Gangplank?

Pros:
Great damage dealer.
Parrrley has slow + dot.
Ult Cannon Barrage makes him useful from any side of the map.
Remove Scurvy is great for getting out of sticky situations and for regaining some health to defend a point.
Can farm well and get the additional gold with Parrrley.

Cons:
Quite squishy.
Doesn't really have an escape.
Armour can counter but in Dominion, not too many people build so tanky.

Ghost is crucial for getting around the map in Dominion.

Ignite has often got me a number of unlikely kills. Or kills when someone is running! Always handy for those Trynd's.

The only other option I may use is Exhaust. But not knowing the opposing team make up, ignite is always the better choice.

Marks - Armour Penetration - Greater Mark of Desolation
Seals - Critical Strike Damage - Greater Seal of Critical Damage
Glyphs - Critical Strike Damage - Greater Glyph of Critical Damage
Quintessence - Armour Penetration - Greater Quintessence of Desolation

Other options:

Marks/Quintessences - There is no real reason to take anything other than Greater Quintessence of Desolation. By far the best option for these slots.
Seals - Critical Strike Chance - Greater Seal of Critical Chance
Glyphs - Coolddown Reduction - Greater Glyph of Cooldown Reduction

You should be able to build enough crit to not need the Greater Seal of Critical Chance, but if you are wanting ~98% crit chance, go for it.

Greater Glyph of Cooldown Reduction runes would be helpful if you are wanting to Cannon Barrage more often to protect points.

Fairly standard attack masteries.

You may opt to take armour over the magic resist. However, I take the magic resist as I tend to encounter more AP characters.

Grog Soaked Blade:

Gangplank's basic attacks apply a poison debuff for 3 seconds that can stack up to 3 times. Each stack deals (3 + 1 x Level) magic damage per second and slows the target by 7%.

This coupled with Ignite has often given me many kills when someone is trying to run and escape.

Parrrley:

Gangplank shoots an enemy with his pistol, dealing 20 / 45 / 70 / 95 / 120 (+1.0 per attack damage) physical damage. Gangplank gains 4 / 5 / 6 / 7 / 8 extra gold if Parrrley deals a killing blow. This ability can critically strike and applies on-hit effects, including his passive.

Gangplanks number one damage dealer. Gives a slow and dot and procs on-hit effects - combining with our Sheen part of Trinity Force. Crits for insane amounts of damage!

Remove Scurvy:

Gangplank consumes a large quantity of citrus fruit which removes any crowd control effects on him and heals him for 80 / 150 / 220 / 290 / 360 (+100% of ability power).

Amazing spell for getting out of tight situations. Also provides a nice heal which can be used to prolong staying at a point to defend.

Raise Morale:

(Passive): Permanently increases Gangplank's attack damage by 8 / 10 / 12 / 14 / 16 and his movement speed by 3 / 4 / 5 / 6 / 7%.

(Active): Gangplank fires into the air, disabling his passive boost but inspiring himself to gain an additional 12 / 19 / 26 / 33 / 40 attack damage and 8 / 11 / 14 / 17 / 20% movement speed, with nearby allied champions receiving half that amount for 7 seconds.

Maxed last as it isn't as beneficial as the other skills, but provides a nice boost in attack speed, attack damage and movement speed. Also 1 free point in Dominion.

Cannon Barrage:

Gangplank signals his ship to fire upon target area for 7 seconds. Each cannonball deals 75 / 120 / 165 (+20% of ability power)magic damage to all enemies hit. Enemies caught in the area of this ability are slowed by 25% for 1.25 seconds, with the duration refreshing if one stays in the area.

Great skill for interrupting point captures (5 points) and giving your team a chance to get there to defend it.

Slow can be handy for those enemies trying to escape, as well as doing decent damage.

I take one of each skill - Parrrley, Remove Scury, Raise Morale,at the beginning to get out of any nasty CC and to give my team-mates a small speed boost at the start.

Max out Parrrley first. This is Gangplanks most lethal skill.

Take Remove Scurvy when you cannot take Parrrley and Cannon Barrage whenever possible.

Cannon Barrage is a great tool for preventing those annoying backdoor caps or if bot starts going down.

Raise Morale is maxed last.

I build a tonne of crit. These really make good use of the crit damage runes.

Opening with Sheen gives you that little extra pain inflicted by Parrrley.

To help with getting around quickly I take Boots as soon as possible if safe.

Once the initial scrap has settled down, I build Boots into Ionian Boots of Lucidity. These help greatly with making use of Cannon Barrage.

Building into Trinity Force gives great all round purpose - movement speed, slows, crit.

Taking a little bit of life steal with Vampiric Scepter to help farm some hp off minions while defending.

Infinity Edge is amazing on Gangplank. No more needs to be said why I take this.

I finish off my Sanguine Blade before building more crit with Phantom Dancer and Atma's Impaler.

Please comment on my build. Being my first one, I am unfamiliar with describing everything and adding things people would like to know.

Enjoy the destruction of your opponents!

I Hope not to be on the receiving end of anyone using this guide :D