Why Gangplank?
Well, he has great Dmg output, even though before you have a lot of crit-chance it is a bit random how much dmg you do.
He can Buff Him self and allied Champs. That Buff alsow gives him movment speed, so he can flee pretty well.
He can Heal him self for an big amount!
He has an Global Ulti, that slows and deals Dmg.
In late game he just kick´s ***!

Why Crit?
Thats Pretty easy to Answer: His main DMG Skill "Parrley" can Crit.

For Example at Skill lvl 5 Parrley deals 150 Dmg
Plus 100% of his Attack Dmg, lets just say that would be around 100 (normaly more)
So that would be 250 Dmg (non Crit)

This Skill can Crit, so when it Crits you do about 500 Dmg.
If you have Infinity Edge already it would be something arround 625 dmg.

This skill has an cooldown of about 4 sec, so it is nearly Spam-able.

thats why you should play Crit-Plank.

Pros:
-Great Damage output
-Good Harassing ability´s
-Good Support all over the Map with ulti
-Nice Buff witch gives us attack and movemend speed.
-Can Heal him self and stay long in lane

Cons:
-very Squischy
-Focused first a lot
-Slight mana Problems
-pretty vulnable in early game

Glyph:

I have chossen some Glyphes for cooldown reduce. But you can alsow use Some Glyphes to increase your Crit chance or some for ability power to push heal a bit. Its all up to you, what ever you prefer.

Seal:

In my opinion its pretty clear why to take those. You just need some Manareg-Push, simply because you can stay longer in lane and you can Anoye your enemys more. Shure you can use alsow Crit chance increase seals here, but i dont recomend this.

Mark:

Here you can ether choose Crit increase, like I did or you can simply use Amor pen. Both is Possible and both works out great. You have to choose what maches you best.

Essence:

Here you can choose some more. You can use Crit increase, Health increase or Amor pen.
everything is nice, i choosed Crit increase, but as always it´s in your hands you can choose what ever you want.

Well I Honestly dont like it to say Wich items you should take.

You should defenetly get shen! This item is just perfect for Gp. Just use an Skill like your heal or your Buff to Aktivate its passive (100% base dmg at the next hit) then use Parrley. Parrley does not aktivate Shen, but shen puches the dmg of Parrley.

Of corse there are some You should defenetly dont take with Gangplank, such as:
-Items who give you Ap (Rabadons, etc.) pls dont take those to push your heal or dmg of ulti.
-Items who give You mana (Tear of the Goddess), Yeah you have some mana probs with Plank, but they are not as worth as it seems, so pls dont buy mana items.


The items I mentioned in the beginning are great for Gangplank, but of course you can change them to your own condition.
Items you can alsow Use:
-Boots! I Recomend the cooldown reduce, but you can alsow take some for attack speed, armor or
Movemend speed, thats up to you.
-You can even Buy something for more survive ability if you want. Like Warmogs Armor.
This all is up to you!

First his Passive, Grog Soaked Blade. It poisens your enemys and slows them. The slow is great to stress up your enemys. Yaou can save your alies with it or your own *** as well. Alsow the enemy wont flee that easy from you.
Second: Parrley. This is your main Damage Skill. As I mentioned already it does a good DMG output especialy with Crit items. BUT DONT ONLY USE THIS ABILITY TO DMG YOUR ENEMY`S!! Gangplank is a Meele-Champ. Go an get them feel your Swort as well! Alsow you can use this skill to Farm. If you last hit with it you will get bonus gold. So try to last hit minions With it.
Next is Remove Scurvy. This skill is just perfect! It removes any growd control effect on you and it heals you pretty well! This saved my *** so many times.
This is a Nice Buff: Raise Morale. Increases your Movement speed and your base Attack. Since the last Patch on Gp its not necessary to kill a Minion any more. Use This skill to hunt fleeing Champs down; Close the distance to an Target or just buffing your allies.
His Ulti. Not pretty much to say here. It slows your enemys and is usable all over the map.
Use it to Support your alies or just let it rain bullets in a team fight.

Here I choosed Flash and Exhaust:

Flash, becourse of its great uses for catching up and fleeing.
Exhoused, for autoattack-Champs, such as Xin Zhao, Master Yi and Trynda. You can stop them Pretty Well with this.

Inseat of Flash you can alsow useGhost, Thats up to you.

Wich Spells not to take:
-never take Clarity! Once more, his Mana problems aren´t that hard.
-alsow dont Use:
-Revive


-Fortify


-Rally


-Smite


-Clearvoance

In Lane:
-Try to last hit minions with Parrley.
-After Lvl 3, Start Harrasing, first Use Raise Moral, for Attack and closing distance, then
use Parrley
-Try to Help out with Your ulti When ever you can, but be aware to not steal a Kill.
-Enemys can hardly flee from Plank, becourse of his passive, that slows enemys he attacks.
-You can help your mates to survive if you attack the oponent to slow him.
-As soon as you get shen start with serrious Attacks:
-Aktivate Shen with either "Remove Scurvy" or "Raise Moral"
-Then Atack your enemys with Parrley and Auto attacks.
(remember shen has an 2 Sec cooldown!)

Alsow dont forget, Gangplank is an meele Char, so dont only Use your Parrley to Dmg you oponents, you can also normal Atack Them.

There is nothing more to say about laning, it´s Pretty easy with Plank.

Just be aware of some special enemys, like Sion or Tarik, who can long distance stun. Alsow be carefull with Xin Zaho, Master Yi and Jax, who can Jump or Charge on you.