To anyone who mains gangplank, how do you build him to maximum tanky dps efficiency lategame? Is it better to go full tank + trinity force or get atmogs + damage items? How important is lifesteal late game? How many gold items do you recommend getting?

I would say it's situational in most cases.
However, I build Trinity/Atmogs on him every game, but that's just me. The order that I build them in depends on how well I'm doing in lane/early game.

Everything besides that is situational.

For lifesteal, I enjoy building Zeke's Herald... because it just boosts his utility, which is what makes him great imo... also gives 15% CDR which is ****ing great.

You could always try metagolem.
Can't go wrong with it.

Trinity
Atmas/Maw of Malmortius
Warmogs
Mercs
Situational (1)
Situational (2)

You can build pretty much anything on him. You can make him an aura bot if you really want, because he can't really get close enough to an enemy carry to kill them like Riven or Olaf would, and he doesn't really have the peeling power of Nautilus or Alistar.

Randuins Shurelias + some damage.

Atmog is a must on Gp I reckon, replace atmas with maw as Maid said if you must but I never do
Boots can be: Mercs, Ninja or Cool down
Trinity + Infinity Edge if you want some good laughs over your parrley
Gp is so versatile that your last item depends not only on the enemy team, but your's as well-namely, what is your role?
If you're carrying you might need LW/BC; otherwise build according to enemy team.
Top picks:
Randouin vs melee
Guardian Angel for surviving (meh cos you dnt have much lifesteal),
Frozen mallet (again meh this time because you already get slow from passive+Q+possibly red buff)
Blood thirstier for sustain
Thorn mail against AD
Banshees is not actually needed unless youre facing AP sion/Veigar (lets you save oranges)

I start with a Manamune, as I tend to have mana trouble all game. Last-hitting with Parrrley is easier if you have plenty of mana to do it, and you always stay high enough on mana to help in fights or use your ult from afar. The boots you want are Boots of Lucidity, because it's really geat to use Parrrley more often and your ult. The 2nd main non-boots item I build is Sheen, for obvious reasons, but hold off on Trinity Force. I usually get Warmog's next so you're not quite so squishy. Then you can upgrade your Sheen or get Infinity Edge next, doesn't matter. Then do the other. Then get Phantom Dancer. Now, if you get extremely fed, late-game you can sell your Manamune to buy a 2nd Infinity Edge, giving you over 100% crit chance with elixirs. IE also gives slightly more AD than Manamune, and late game you won't need the mana as much.

Note: This is only for solo top, which I mostly do. If I jungle, I'll usually replace Manamune with Wriggle's because mana sustain isn't as important and Wriggle's is. Same rules apply; sell at end for a 2nd IE.

I wasn't really going to give my opinion/build on GP, but I saw that everyone else got voted down hard so I thought I would. :P I'm sure it'll get voted down, but meh. It works for me. To each his own.

Wow, Triforce/Atmogs on GP gets downvotes? I can't wait to see what those pros build instead.

I mained gp for a while back when his q costed less mana. You could harrass all day and get a trinity and rape your lane.

However in the current meta gp is a little weak up top. No sustain besides mana costly oranges, and worse base stats than most other tops. Harrassing a udyr or ww with parley is pointless, and you will get zoned pretty easily. To compensate I always built wriggles for the free ward, lifesteal and armor. Get merc treds so you are un-cc-able.

Here you have a choice. Get trinity and go tanky atmogs. Or build a ie into an atmogs. In the end you need the health to survive, crit chance is always good with parley, and your wriggles will keep you afloat(lol pirate pun) in close team fights.

As other people said, against ap heavy teams get a maw, if you dont have another tank on the team get a randuins instead of a atmas. If you get fed early in lane, getting a sheen can allow you to zone almost anyone, gives you more mana to q with, and this sets up the trinity build. (this build is better for poke team comps as your q will hit hard, but not much else). Or get the ie which is more well rounded in team fights and 25% of the time your q hits like a *****.

Suggested build:
1) boots three pots
2) wriggles level 2 boots
3) ie if you are up warmogs if you are down
4) opposite item from step three
5) maw/atma
6) either opposite item from 5 or a randuins/thornmail if they have lots of ad