Gankplank is a champion, who is really weak in the early game, especially against some OP Top Champion's. He doesn't really counters the most played Topchamps, but that mustn't be a problem.
You have to concentrate on farming for the first 5-15 minutes and play defensive (exempt your jungler babysittes).
Farm is very important, because if you got items like Hydra and STATIKK SHIV, you will have a great farmspeed to have a much more bigger goldadvantage, than youre enemy. So you can start to play more offensive and try to kill your enemy.
You just need some patience.

In my opinion Gankplanks power is for the most League of Legends players unknown.
He makes often a impression, for people who don't know how to play him, as a really weak champion, who has no impact.
Because of that I classify him as one of the most difficult Champions.
First and foremost you have to care with him in the early game, what is not really easy, in comparison to other bad early game champions.
But if you got the right items, Gankplank rules his lane and the teamfights if you know how to do.
His high crit damage, mainly with the Q, which has a low cool down, arranges for a extrem high damage-rate in teamfights. The special thing is that he does not only hit 1 enemy, but also the whole enemy team, because of the Hydra and STATIKK SHIV.

Focuss on the Q, in early game to have a faster and higher goldexploitation.
Use the W , if your enemy is a range-champion, or a champion with much CC .
The E could help you to be more flexible, e.g. to escape better from ganks or to push you lane faster.
At least the R , is usefull to escape from ganks, to win in a 1vs1 fight, or to help your confederates in each situation. So keep your eyes over the whole map.

Raise your Q up high, and try to last hit the minions with it as much as you can. The Avarice Blade helps you to gain more gold, so get it directly after your boots.