Despite common perception, Gangplank can be one of the strongest melee dpsers in the game. His Parrrley counters healing on any target he focuses, his Parrrley hits like a truck mid and late game. Combined with his built in cleanse (yes it works on suppression), Gangplank can navigate the battlefield and obliterate even well guarded carries.

Pros
-Great Farming Capabilities
-Free Cleanse/Heal
-Has a ranged harass on a melee champ
-Global AoE Ultimate
-Supports other physical carries well
-anti-heal passive***

Cons
-requires a lot of farm
-no stun
-very specialized role

Marks - Greater Marks of Desolation: The armor penetration is hard to acquire without sacrificing damage in game and helps farm creeps with Parrrley early on. A common alternative is critical %/damage runes. I find that the critical strike route turns gangplank into a one hit wonder, making him much easier to shut down.

Seals - Greater Seals of Vitality: The HP/level runes in addition to the bit of health gangplank acquires from Trinity Force make him sturdier throughout the game. Acceptable alternatives include armor, mp5/level, critical strike.

Glyphs - Greater Glyphs of Clarity: Using some form of mp5 runes is essential for Gangplanks ability to farm using Parrrley. Due to the extra gold every kill with Parrrley, each additional CS provides a hefty gold boost to GP.

Quintessences - Greater Quintessences of Alacrity: In contrast with most of the other runes, I put the Quints towards Gangplanks late game DPS. Using an alternative build, these runes would prove less useful. However, 10% attack speed makes an enormous difference in GP's ability to deal damage once he gets his IE and Bloodthirster.

Recommended:
Ghost - With a passive/active MS buff and a cleanse, Ghost is great for chasing, escaping, or navigating a battlefield to shut down carries.
Exhaust - GP's soloing ability is severely limited early game and Exhaust provides both a slow and lowers enemy damage.
Flash - Depending on your play-style, flash can provide an alternative way to move around a battlefield and reach enemy champions.

Alternative:
Teleport - The extra map presence and mobility can make GP a strong pusher. Early game, it allows him to farm faster, and late game he can push, and still assist his team with an ultimate.

Ignite: I HIGHLY recommend avoiding this spell, as Grog-Soaked Blade already provides the same effect and applies every auto attack. However, if their team has 2+ healers it could be a viable spell, still, i recommend letting one of your teammates pick it up.

Start with a Sapphire Crystal and 1 of each potion. I recommend a solo lane if you can get one, but don't take one over a strong carry. Don't waste mana harassing champions early. Simply aim for last hits with Parrrley. This will be critical for amassing gold to get to Gangplanks late game as fast as possible. Unless forced to back earlier, save up for a Sheen, rank 1 boots, and a few potions. Also try to throw in a ward if you have a little extra. Keep in mind, Gangplank can push very easily, and overextending is a great way to die a few times in a row.
Gangplank can put out a significant amount of damage for early ganks, however, the his most important task is to apply grog soaked blade to lower healing the target can receive. Exhaust can also be useful for ally ganks, as well as escaping from enemy ganks. Once Gangplank reaches level 6, his ultimate can be great for scrapping kills, defending towers, or initiating ganks.

Mid game strategy on Gangplank closely resembles early game. The focus remains last hitting as many creeps as possible with Parrrley. Save enough mana to use Remove Scurvy for easy escapes. Maintain keen map awareness, as GP's ultimate can prevent a gank from the other teams jungle or provide a nice slow to ensure a kill. However this is NOT its main use. Cannon Barrage is, in my opinion, the best skill for protecting towers in the game. It not only quickly kills off the creeps, but also slows any attacking champions forcing them to back off early to avoid taking tower damage.

While GP will want to remain in his lane most of the time, until he has farmed enough for infinity edge. At this point, his parrrley will hit extremely hard, and he will be able to gank other lanes easily.

Making it to late game without feeding is crucial. Even if you are slacking off in kills, you should have similar items to someone who is fed on the other team. I typically have around 150-200 CS 20-25 minutes into the game which enables GP to have 3-4 finished items. At this point, GP can control teamfights with his moves and harass players before they start very effectively.
The 2 essential tactics to keep in mind with a late game GP are: 1. Constantly harass anyone who gets in range of Parrrley, it should have around a 4 second CD, at hit for 400/1000 noncrit/crit. 2. Don't blow Remove Scurvy as a heal unless you are CERTAIN it will be up for the next team fight. Remove Scurvy not only enables to you safely escape long CC, it will also give you the ability to soak up a CC like Ashe's arrow instead of a teammate, who wont be able to cleanse and heal through it.

Once you begin ranking Raise Moral up, GP also provides an enormous speed/damage boost to nearby allies which significantly helps your team out. Use this skill as soon as your team initiates to help get on your targets faster and the burn them quickly.

Also, if you get enough gold to buy a Phantom Dancer outright after the rest of your items, replace your boots, and it will help your damage output a ton.

While GP lacks strength early game, his farming capabilities are off the charts. By avoiding wasting precious mana harassing enemy champs, GP can receive 1.5x as much gold as any other champion simply creep farming. His cleanse makes him much more difficult to gank, and simultaneously his Grog-Soaked Blade and Ulti make him perfect for ensuring a kill when your jungle ganks. As long as Gangplank survives till he acquires his late game build he will have no problem dealing with anything your opponents throw at you.