To sum it up in one sentence, you have to know how to play really fast with G&W.
He takes a high learning curve in high level play. You have to have intimate knowledge of baiting, and knowing when to throw out pokes. It's important to learn to not be limited by G&W's best spacing moves alone, but to also learn to use his fast moves to get inside range. Don't spotdodge mindlessly, since the only reason to spotdodge with G&W is to get grabs since the rest of his options are kinda too slow or aren't GOOD enough to warrant the risk of spotdodging. Jab canceling is a must with G&W, as well as Bucket Braking, and learning to use F-air and B-air out of shield is extremely valuable. If you can powershield a lot, then props to you, because G&W is basically benefits more than anyone else from powershielding (besides Luigi).
Oh yes, and juggle as much as possible. G&W is one of the safest and most powerful jugglers in the entire game, and learn to abuse charged Smashes since while his Smashes have high startup, they have quick releases, and they are safe on block.
In general though, G&W is a character who thrives on being on the move constantly.
