i would like to hear what some of the other players have to say on this... i have trouble with him too.
There are a couple of things that I have found, though... but i just want to say that I dont consider myself an elite player, so others could probably give better advice.
Heres what I have found that seems to work. Anyone should feel free to comment or correct me if im wrong.
1) His backair is rediculous, and has high priority. However, since it is a back air it is a little more predictable (he has to be facing away to do it), and if you are watching for it you wont get hit with it as much. If he is using it alot, you can use a tornado to go right through it, or try to shieldgrab it (but keep in mind its multiple hits... gotta wait for them to finish or youll get hit with the last hits)
2) His downair is also rediculous, and will knock you out of your tornado if he hits you in the center with it. Any time he does it and youre using the tornado, make sure he hits you in the upper side part. not the center. That way hell just get sucked in. Also, he has little/no ending lag after the Dair, and can smash or tilt directly after landing. This means you have to have perfect timing if you want to punish the move. (think of it like MK's glide attack... you CAN get punished after, but most of the time your opponent rushes in just to get downsmashed) Alot of g&w players seem to bait you into punishing the move, then smash you in the face when you move in for the kill. Watch out for this. If you cant get the timing right, it might be better to avoid it.
3) I have found it beneficial to be a bit more cautious when playing game n watch. He has no projectiles other than his frying pan (which is easily avoided... also, you can often sneak under them and ftilt him while hes doing it, or drill rush right when he starts it up, it will often make it through), and he cant really force you to be the aggressor, all the time. DO NOT run in and try to punish his Fsmash... He will just keep spamming it, and you will run in and get blazed up. Pick your shots, and bait him into making mistakes. Try to maintain pressure without going ape****.
4) I have noticed that he will often drop below the stage before using his up b to land on the stage. That is the moment to shuttle loop (you can often hit them without leaving the stage). Mix it up with dair and fair gimp attacks so he doesnt know which one to expect. I have had little success with the off-stage gimping, though (unless you count shuttle loop)... he often uses that thing that pushes you upwards and I end up waaaay above the stage.
Again, Im not amazing, especially against g&w, but i do play quite a bit and these are a few of the things that have worked for me. Hopefully some of it helps.