In a desperate laning situation, Gangplank has a lot of ability to just stay in lane and farm as much as possible. Parrrley's range and mana cost reduction upon last-hits allow Gangplank to farm even with the enemy efforts to completely zone him, building gold per 10 items let him stay in the game despite the enemy controlling the lane, Cannon Barrage and Raise Morale can be used to quickly aid allies even if you are pushed to your turret, and Remove Scurvy sustains you against enemy harassment. This strategy would prefer maxing Raise Morale for the attack damage bonus to better allow you to last-hit under your turret, with Remove Scurvy as necessary to sustain and stay in lane. Leveling up Parrrley first in this situation is unpreferable due to the mana cost increase while the gold bonus increase is less useful than simply casting Parrrley again and gaining another last-hit, or having more attack damage from Raise Morale to gain more last-hits underneath the turret while Parrrley can be on cooldown.
Raise Morale can be used defensively and offensively, to be able to move fast for both, and have extra damage for the latter.
Use Raise Morale when pushing a tower. This can quickly give you and your allies the boost you need to take down buildings, effectively allowing you and your team to rush towers and your enemies not having enough time to defend it.
Cannon Barrage can be used across the map. You can help out in ganks/teamfights elsewhere, by assisting with the damage and massive AoE slow.
Paying attention to low health enemies on the map can land you a surprise kill with Cannon Barrage.
Try placing Cannon Barrage in the path of fleeing enemies to cut them off.
Cannon Barrage can save a turret surrounded by minions, with some luck.
'Tankplank' builds are very effective, offering survivability and a chance for Gangplank to fully utilize his skillset. It also allows Gangplank to increase his DPS as he builds towards an Infinity Edge.
Gangplank is effective at jungling, though some runes are required to complete it. Jungle Gangplank is based upon building Spirit of the Elder Lizard and using Cannon Barrage to gain bonus gold by assisting lanes without ganking. This allows Gangplank to spend a significant amount of time farming the jungle without missing opportunities to score assists or kills thanks to his high mobility.
This is only recommended if you can keep up with your build. The extra 2100 gold cost may slow your build down early game if you can't farm up well, or if your lane opponent gets the advantage.
AP Gangplank, while an unconventional build, can make him difficult to kill and add a surprising amount of damage to his Cannon Barrage. Lich Bane effectively adds a .75 AP ratio in compensation for lack of crits and his Remove Scurvy will heal for much more. AP health items such as Rylai's Crystal Scepter and Rod of Ages would also help his survivability.
Laning against Gangplank can be a bit frustrating. Parrrley has a fairly short cooldown and deals a lot of damage at early if it crits. Starting with Cloth Armor and 5 Health Potions will give you a lot of sustain and damage reduction to deal with this.
Champions with multiple crowd control abilities, namely Blitzcrank, can significantly slow down Gangplank, despite his Remove Scurvy. Since this ability only removes the current debuff, one can use a "soft" CC ability (such as a slow) to force him to use Remove Scurvy, then utilize their "hard" CC (such as a stun or suppress) to really begin harassing him. Also keep in mind that AOE or constantly-reapplied CC (such as Omen of PestilenceYorick or Make It RainMiss Fortune) are reapplied immediately after being removed, so with Remove Scurvy's cast time ends up actually taking longer to get past. Airborne CC (knockups/knockbacks, pulls) cannot be cleansed, so CC from Alistar, Aatrox, and Blitzcrank are not countered.
Although his early harassment is strong, Gangplank is fairly squishy in laning phase. Putting an aggressive pressure on his lane will weaken his strong mid/late game.
Parrrley does average damage early game, but it will become quite effective late game, once he has built crit chance or on-hits. Try making him build defensively instead of desirably towards offensive output.
Cannon Barrage has a huge AoE and global range. Quickly avoid the AoE if he uses it when you are escaping. Be aware of its potential to appear at any time/location.
Countering Gangplank requires starving him of his farm, get an early advantage by pushing him out of the lane, and forcing him to play with more risk than he would usually like. Teams should also consider prioritizing him as a target for ganks and in team fights depending on his build and his allies.
Being aware of his Remove Scurvy would give you an advantage if your kit has enough [CC] compared to the ability's fairly long cooldown. Casting a minor CC skill would force Gangplank to use this ability and press your damage as much as possible within the interval of his Remove Scurvy.