This is a really fun way to play gangplank, as adc, you'll find yoursel doing somethink like 1k dmg every "Q" Mid game, later even 1,5k. Usually the adc is ranged, we can consider parrrley actually as a ranged basic attack, even if i don't reccomend to play gp as adc in a ranked game, in a normal you could get loads of fun.

The support is very important, the bests are defensive ones, they can give you the hp you loose and you don't have to remove scurvy to get your hp back.
If you don't get a defensive one he will be more usefull after you'll be a little more strong.
Ask if possible, if he can't restore mana, to get clarity, this will help you a lot keeping you in lane longer.

Early game, untill the first time you shop, don't go agro, just keep farming your minions, use always parrrley when possible to kill creeps, this will restore half your mana and grant you a great farm. Be sure you can kill the minion, or you'll find yourself without mana very fast.
Even if you could don't use your q on enemies, it is difficoult that you get the critical, and much mana will be lost.
The 2 mana pots will help you to restore the mana you are wasting to kill creeps and to remove scurvy.
When you are too low hp and you have no more mana, just go shop and teleport back.

Now you should be lvl 6, nearly, or even more.
Your Lvl and even the sapphire crystal, if you had a good farm you should have it, will grant you enough mana to spam your parrrley like crazy even vs enemies, when you'll be out of mana just go shop and teleport back.

Perform like an usual adc: stay away from enemies and blow them up with your q, during teamfight focus enemy adc and Apc to win, even going melee if you need.

You'll find yourself so fast you wont need boots, it's useless to spend 1k for something you don't need. Your movement speed will be good enough with zeal and then trinity force you will be 400, with raise morale much more.

Spam your "q" to last hit minions, don't use it vs enemies early game, the half mana cost is refunded if you kill minions, be sure you kill them with the parrrley blow or you will loose a lot of mana.
Sheen and trinity force influence his damage.
Remember that parrrley actually is a ranged attack, with all pros and cons.

It is the reason i prefer 2 mana pots instead of the healt ones:
His cost is 65, 1 mana pot restore 100mana, 1health pot heals 150.
At level one is better one healing pot, but at level 2:
The cost remains 65, but it heals 150 like a pot, and you have all the pros of removing cc, which will help you a lot when you are ganked,
The cooldown is high, this means that if you are ganked when it is on cooldown, you are crumb.
Use it only to escape cc or if hardly pushed.

When you use it you stop moving for a half second, pay attention when you use it, if you are running away the enemy could reach you, rather, use it in the moment when the teamfight starts.
Its range is quite good, use it when your jungler ganks to improve speed and dmg, be sure he is in the range, it could make the difference.

Its damage is embarassing, but if used correctly can be very usefull, when your jungler ganks use it behind the enemies, they'll be slowed for a long time.
Check often the minimap to see if somewhere a fight is on, usually you can help top escaping using it behind him, or even if your jungler is ganking top to slow the enemy.
Don't use it to secure a kill, it is quite impossible for you to get a kill this way, the damage is irrilevant.
If you are luck you can steal the dragon, but you must be very lucky.
Of course use it when the teamfight begins, the damage in this case is good, because the enemy team has to remain inside.
You'll feal often to have wasted your ulti, no problem, the cooldown isn't high.

Critical runes are the main point of the build: Early game critical damage will be totally useless, only critical chance, this will make unable to win early game the lane, you'll soon have enough chance to blow them up. and win.
We can say cons:

High expandable rune page: 20k.
Lower early game.

pros

Incredibly strong mid and late game.
More damage anyone could expect.

This incredible damage will be so high that enemies in lane wont back, beliving they can get another parrrley without dying. AHAHHAHAHAHAHAH

I seriously don't know what to tell you. But the guide should be fine anyway. You'll just deal less damage.