This is my first gangplank build. I was goofing off doing a custom game to get a new champ. I used gangplank to juat test him out. I saw others build a physical DPS using Parrrley and moral. So i went and made my own. Anytime ive used this build i've won the game (i've only played 5 :P). I really liked this so im sharing it with you.

I chose Furor marks for the crit strike since its 71.5 % when your done. These + 50% crit damage from infinity edge is a huge boost. Also, you have another 10% increase from lethality.
For seals i took mana replenishment. Gankplank is extremely mana-thirsty early on if you plan on harrasing with parrrley and a raise moral boost.
For glyphs i took attack speed. I wasn't sure what to put here so comment for better glyphs. These just add to this build
For quints i took Armor pens. I suggest Flat health quints or something similiar for survivibility early and late (you are pretty squishy) and maybe somehting for attack.

I feel these are pretty common among heavy DPS champs (yi, WW, Tristana, etc...). I choose crit strike and damage, + damage to minions, increased damage, attack speed and Armor Pen in offense. For utility, i took increased XP gained, more health and mana regen, and a small less time dead. These are good for the heavy DPS build in my opinion but comment for other options.

The best part of this build is the items. Take down tanks with a teammate. i start with a dorans blade allowing me bonus health for the early game ganks. I also get +9 damage with 3% lifesteal. Something is better than nothing in this game. Next i buy a pickaxe since you will probably get a kill/assist early in the double lane. I b as soon as i have enough for it + boots (around 2k but you can get a pickaxe at 1k if you need it). I farm and get kills as quickly as i can (your parrrley should do a good bit of damage). (use moral boost for farming). I get an infinity edge for the attack dmage, crit chance, and bonus crit damage. Next i get a Starks Fervor (best item on this build in my opinion). The lifesteal with the attack speed means someone will have a hard time killing you. I b and get zeal @ 1200 gold then rush a phantom dancer (do what you must for gold even if it means jungling, farming, ganking, tower diving and other things.) I get a cleaver for the debuf on tanks and and the dmage. then i rush a bloddthirster for more damage and the bonus lifesteal.
Other items i recommend are a last whisper, Wits end, or sword of the divine.

The sequnce is pretty basic. Max parrrley first and raise moral second. You'll notice the difference. It might be worth it to put a point into remove scurvy if they have snares, slows, taunts, fears, etc. these champs are morgana, veiger, ryze,sion, taric... you get the picture. This CC effects devastate you (i learned from sion) as you will get pwned. I usually dont if its 3v3 but 5v5 i will put a point in it as its a cleanse on low CD. your ult is top priority. If you need it for escapes, a team battle, a slow to help your teammates, USE IT.

I take exhaust because people seem to run. They usually dont escape when i use this skill. Exhaust + parrrley + ult = kill. Also, this helps in team battles + ganks that aren't working out.

I like to use ignite as sometimes (unfortunately) they run and escape. If i tag em with ignite i always get the kills.

other spells that would/might be worth it would be flash, clarity(only for beginning as late your fine on mana).

If theyre is any others comment and let me know so i can test it out.