Garen as a champion has many problems.
His gap closer is just extra movement speed, and a silence. Sure, you could make the excuse that since it's an auto attack as well you can use glacial augment and frozen mallet to make it hard to get away from him, but really there's a lot that can go wrong before he even hits you.
If your champion has any form of reliable cc, he won't be able to get to you. Even slows can keep Garen from getting to his target. Garen can't just run into the back line. Most successful Garens have to hide in a bush to surprise an enemy - no one wants to be up close and personal with a Garen, so you can't just walk up to them.
Maybe his damage could be toned down a bit, but most people that have problems with him just underestimate him and don't play around him, like you have to do with virtually every champion in the game. Not to mention that CC is usually more important than damage in a teamfight. Unless Garen is instantly deleting every damage source immediately at the beginning of a fight, it's not that big of a problem. If you don't have a way to keep him away from you before he kills you, that's called "Counter-play."
Really, you can't say anything about Garen that you couldn't say about any other tank. In fact, he's probably one of the worst tanks in the game, just because he has no real hard cc. That silence will do nothing to stop Jinx from punching holes in your back line, should they be too slow to get away from her.