So, Hello There!
I've been playing Gangplank for a long time now, and I learned a lot of stuff.
First of all, let me tell you some cool stuff about this build.
I did it once, in a normal game, just for fun. And guess what? It worked!
I do it every time now even in ranked games. Last season i did 71% victories with this build(and some of them with REALLY bad teams.)
This build is called Runnerplank for a single reason. You can catch anyone. And no one can catch you.(Yes, this is just one reason.)

Pros
- Great harass in laning phase, with spammable Parrrley.
- Mobility for escaping, catching, ganking.
- Passive "Frozen Mallet".
- A God of 1v1.
- Good for team fights.
Cons
- He is kinda squishy.
- Bad laning agaisnt long range champs.

I used to play Gangplank with crit damage runes. But then a friend of mine made me see this thread: http://na.leagueoflegends.com/board/showthread.php?t=70173
So you will be delivering more damage when crits comes out, and when they dont. Awesome.

I like to solo with Gp, over mid or top.
Its really not hard to dominate your lane.
Pop Parrrley to keep them behind the minions and last hit as much as you like.
If they come to gank you, its really easy to escape. Remove Scurvy + Flash + Your huge mov speed.

Thats a biggy one. Maybe it doesnt look like much, but your ult does INSANE damage. So in a team fight you want to ult the area, of course, and then focus one especific target (usually the squishy carrier who is better in the game) If your team can keep them inside your ult's range its really great. Again I say, insane damage.

If you are getting trouble with some especific champ, is not bad to make items that can counter them such as:
Banshee's VeilCloak and DaggerGuardian Angel

Gp is much more than the build itself.
He is great for escaping runs, large ganks, small ganks, 1v1, 5v5, catching...
Just find your own way to play him and have fun!
And go get those landlovers!