Rather than discuss how amazing G&W is until I truly believe he is the master of the first 2 dimensions (did the universe implode yet?), I want to go over how to beat him.
Depending on the weight list you look at, G&W is either the second or third easiest character in the game to KO off the top of the stage. I guess my main can be considered Snake right now, so I don't have too much trouble KOing G&W because of his up-tilt. Also, its fun to nade-sidestep the turtle and watch G&W suffer for it. Throw in his heaviness, and Snake really seems like one of the best counters against G&W. But I guess anybody that is good at killing upwards would be a good pick against G&W.
Also, I hear that the "Cypher grab" that works on Snake to eliminate his up-B also works on G&W, Sonic, and Pit. Although G&W and Pit are almost immune to this due to the auto-sweetspot.