Tankplank is an amazing thing. Keep almost your same damage as regular Gangplank builds but have enough health to survive just about anything that can come your way.

Armor pen runes I feel are the best option for any attack damage champ just to get that little extra oomph on your hits.

The mana regen per level yellow runes will keep your mana up along with the meki pendant alone. No other mana regen items will be needed while leveling up and at 18 even the meki pendant will become unneeded.

Cooldown reduction blues are really nice to keep that gun firing at a constant rate and to make sure that you can deny a minion from each spawn wave for the early game to keep your enemies underleveled.

Because this is a TANK build for gangplank and you are going to want to initiate fights, it is better to get into the defense tree so that you can survive. The utility over the offense tree was chosen mostly for the experience increase so that you get even that much further ahead because of denying minions.

Taking a meki pendant and potions first is a must I feel. This makes sure that you will be able to stay in the lane for a while even though you will deny a minion from every minion wave and also throwing out some harass with your gun.

The idea early game is to do one thing... rush that warmog's armor. Getting this item early ensures that you will be able to get it fully stacked before any intense group fights really occur. It is sometimes best to take a solo lane with this build to make sure that you can last hit as many minions as possible for stacks without making the rest of your team angry at you for taking all of the gold of your lane.

Taking mercury treads is a great idea because no one likes being stunned. Gangplank's W ability already removes most every cc effect that you could get on you and so with these you will be the bane of every character who has to rely on stuns/snares/slows to kite you around trying to wear your hp down.

Atma's Impaler is a great item since you will have over 3000 hp with just the warmog's stacked up. Plus the crit chance will be needed throughout the entire game to make sure that your damage is close to the same that a regular gangplank build would have. The armor is also nice to keep those nasty ad champs at bay while you work up your damage items.

Infinity edge. Does any gangplank build actually run without this item? It just screams BUY ME for anyone who ever wanted to play this champion.

Now that you have some damage capacity, why not make sure that those 250% hits from infinity edge happen more often? Also the movement speed and attack speed are great assets to gangplank as a character. Phantom Dancer is a very important item in this build even though it no longer has the dodge on it.

Most games will be over by this point with the other team complaining about how gangplank has too much survivability while doing too much damage and will beg riot for a nerf. The meki pendant you have been carrying around this entire game is now useless because by the time you go out of mana with your runes, you will probably have farmed up enough gold to want to go back to buy an item anyway. This is the point for a bloodthirster. It is a little late to be tacking only your second attack damage item but just trust me. If you get this bloodthirster at this point, it will be pretty much impossible to kill you. Not only will you have more health than most teams can deal with anyway, but now you regen it at a stupidly fast rate.

Take your deny first and sit at the nexus waiting for the first wave to spawn. Follow it down your lane and you will be able to cast this twice before you actually reach the point where the minions start fighting. This typically leads to your lane getting higher level than the other and pretty often leads to an early first blood because of level advantage.

After this, parrrley is what you want to max. This is your main early damage output and harass ability. Also, when you last hit a minion with it, you get some extra gold for your kill and get your items faster than most other champs.

You want at least one rank in your heal ability to make sure that if a jungler with some sort of stun or snare happens to come your way you can make sure to be able to run away. Sometimes, it just seems impossible to avoid all the harass that your lane pushes out and so it is always nice to have some way to regen a little hp.

Pros:
Close to the same damage as regular gangplank builds
sets up early game kills even if across the entire map
survivability of any other tank

Cons:
mana can be a little ****py at the early game
damage not as high as regular gangplank builds early game until items come

Last hitting is essential for tankplank. Early game you need to rush that warmog's and so don't be afraid to shoot one in the face with your gun to get the bonus gold it gives. Even after buying the warmog's, you are going to want to last high even more. Last hitting after buying the warmog's builds up your max hp for the rest of the game and the faster this happens, the better. Tankplank relies heavily on what items he has in his inventory and so the faster you can get them, you will start to kill your opponents faster and eventually they will just surrender because you are too highly fed to deal with you any more.

You are going to be the initiator as this build. You are a tank, after all.

Early game after you get your ult, watch the map for potential kills. Always drop your ult if you see your team get into a fight to either help them get away or help them get a kill. The assist gold is worth it.