I'm pretty sure these are two of G&W's worst matchups ;_; and I haven't quite figured them out myself.
Against Marth, I would suggest NOT picking Battlefield or Yoshi's Story. His sword works too well with the platforms there. Again Peach, I'd avoid Fountain of Dreams because dsmash on those low platforms will eat G&W alive...also not Dreamland or Mute City because Peach survives forever there. I can't really think of a counterpick, as long as it's not those stages lol.
Against Marth...hmm I'm not sure. You're gonna have to WD around a lot and try to find an opening. I would mostly start with an fair and try to combo from there. Fair to dtilt, mostly. Try to cc dtilt if he does a weaker attack. Stay out of tipper range for sure! Edguarding Marth, I'd either be aggressive and jump out there to hit him (nair, fair, or dair) or wait for him to up B then hit him with a fair (stay away from the edge when he up B's, then get close and attack).
Peach it pretty hard. You can't CC anything because dsmash is so good against G&W! This means you will have to avoid using his awesome dtilt
...You may have to play a hit-and-run style against Peach, you probably won't combo often. Bair works so well against Peach, use it often! Also try to chain a few fairs...if you're going to fair make sure you hit with the stronger part of the move (like soon as the move comes out). If you hit with the weaker part of the fair she can just cc dsmash. Edgeguarding is quite pointless lol, but if you think you can land an nair go for it. When recovering, try to sweetspot, and be ready to tech the dsmash!
Also, most Peach players are super predictable so you might be able to find an attack pattern...if you see a pattern just wait for it and punish it. For example, if they like to bair into dsmash, then avoid the dsmash and punish her in her lag. Not the best example but that's the only one I can think of lol
