Gangplank is a very shabby character. His ult is almost useless (yes, you heard me) due to the random factor (I've honestly walked through one of those without boots at level 6 and didn't get hit. Lucky? Maybe. But it doesn't deal that much damage or slow that much anyway.) He has the only ability that denies your creeps, but eh, it's still pretty weak. His Q and E are pretty alright, but that basically makes him a two-trick pony.
Garen, on the other hand, is ludicrously good early-game, and all of his skills have a decent use.
You have a gangplank that's going critical with phantom dancers/infinity edge and he does far enough damage with his normal attack, being able to critical for around 800~+ with one move that has a fairly low cooldown is just a bonus, he doesn't suck, if you played with a bad one then that was the player not gangplank.
I don't know if you play in practice games with level 5's but any avid player of LoL knows that a properly built gangplank can tear mostly anybody apart in a 1v1, and that's when he is not fed.
Let me put it this way, no champion is bad.