Gangplank is by far my favourite AD champ. I tried many ways to play him and I came up with build shown above. And I am finding it to be the best build I tried so far, it combines high survability with very high dmg and some nice support + CDR capped. Of course build above is its perfect version, but it is not impossible hard to get all these items in real game. Last time I got all of these in 40 minutes game having 11/3/18.
Of course buying order may vary depending on situation. For example I sometimes like getting Ghostblade earlier or IE earlier. Like I said depending on game. In general I like the idea of getting surv items (Shurelia, Phage, Atma's) earlier not to get killed so I can farm better with purpose of rocking later.
I am using 3x crit chance quints to help me get some nice crits early game. You can swap mana regen seals for Armor ones and blues for MR ones- they are great too.
I use pretty strange masteries setup. It is because I dont find shorter summoner spells cooldown to be so important on GP due to having cleanase like skill (remove scurvy) and ghost like spell (raise morale).
The spells I am using are ghost and exhaust. Ghost is great combined with scurvy- you can get out of any situations. Exhaust is great when engaging in 1v1 or making them stay longer in your Ultimate.

THE WORK IS STILL IN PROGRESS OF COURSE! YHAAAARRRRR

P.S Sorry of my EANGLISH!