I'd say people overestimate how hard fighting a good GaW player can be.
People say "spam Shuttle loop and space Ftilt" but a smart GaW can start baiting shuttle loop, and if you miss, you're either going to die, or get Usmashed because of his Uair which pushes you up and gives him time to whip up something cruel and unusual to punish you with.
Both players have to play very defensive in this matchup, and if either player chooses to play aggro they get *****. It's bad to have GaW underneath you on a platformed stage, as his Neutral Air ***** anyone landing on platforms, and his Uair can keep you up there until you run out of jumps. Its best to DI off stage and recover toward the ledge, but watch for a Fair in this position as you're likely to die if you get chased off stage this far and hit.
Just be patient, as you slightly outrange GaW and he's going to have to work harder for a kill than you will. Be patient and don't use Dsmash to kill before 90%, mainly because even if they DI badly, bucket braking will still save them, and make future kills more frustrating. Shuttle loop is unlikely to kill, as you'll be using it a lot defensively. Try and get a Fsmash in if possible.
Utilt beats Key, though I'd prefer to just Powershield and Neutral Air or shieldgrab. Utilt will kill GaW at 110% ish though, so keep that in mind.
Band Final D. You can fight equally gay as GaW on Brinstar or Cruise, but if you get Uaired on Final D you're gonna feel some pain (Slikvik 9d Me after I got Uaired in a Money match, luckily I have African DI and was at 0%).