I just got gangplank and i love playing him. All his abilities are so useful and awesome. My current build is:
Boots (depending on enemy team i choose Merc or Tabi)
Infinity Edge
Warmogs
Atmas
2x Phantom Dancers
I usually start with cloth armour and 5 pots. On my first back i buy my avarice blade, lvl 1 boots, pots and wards. Now i get 10 extra gold for each minion which is awesome for farming. After that i rush my infinity edge, getting the bf sword first. After ie i rush warmogs---> upgrade avarice blade to atmas. Then i get my phantom dancers for that crit chance.
In the end I have 100% crit, pretty high damage, high health thnx to warmogs, really high attack & movement speed. In teamfights i just run up to the adc and shoot/slice him to death. If they try to stun me, i just use oranges and continue killing him.
My question is: what do u think of this build and how do you think i can improve it. I think the build needs some lifesteal and armour pen but i don't have enough space for it.

Pretty good build i would say Get rid of a phantom dancer and grab a Stattik Shiv the magic damage will prove helpful. Also i would lose the boots(My Prefrence) and grab a youmuu's this gives u movement speed (when needed) and armor pen plus some extra AD. As for the other parts of your build Thumbs up. Oh and if not the youmuus grab a Hydra. if u get a hydra i suggest keeping the boots since u wont have the activatable movement speed. Hope this is helpful

100% crit without armor pen will net more damage overall than a lower crit chance with armor pen. However, if you're over 50% crit chance, you might as well split the difference - especially since it takes very specific items to hit 100% crit.

However, not having lifesteal makes you extremely vulnerable, and if the team starts rockin thorns, you'll end up killing yourself. You won't be mitigating much of the magic damage, and the 30% magic damage return is based on your full damage before mitigation. In other words, if you hit someone for 1000, you'll take 300 damage, but they may take 500 or less from the hit depending on their mitigation.

Before they changed the sword with grieving wounds, it was possible to have 10% lifesteal and still hit that 100% crit chance.

These days, you're better off fitting a BT - the last whisper is situational since the meta appears to be moving to health stacking with resists lacking their defensive utility this season.