As it happens, I was just playing this match with Xelic earlier, and it's not rare for us, so I'll tell you my thoughts on it.
If the Peach player is anywhere near your skill level, at all, you shouldn't be able to win this match.
And that's all. G&W is a really bad character, and Peach is excellent at showing you exactly why that is. G&W's skill isn't in fighting on the stage, it's in edgeguarding, which he does rather well. But he can't edgeguard Peach, and if he tries, he'll probably get KO'd. Conversely, Peach can edgeguard G&W far more easily/safely than most characters can. Just a simple turnip can knock you 6 feet from the edge, and that's enough to prevent you from being able to recover. And she has plenty of other huge advantages. This match, for me, is much harder than Sheik or Falcon. I'd probably set it as G&W's... third or fourth hardest, assuming Falco and Marth to be above her for sure (some people say G&W does well against Falco, but I assure you, that isn't the case).
What can G&W do to hurt Peach? All G&W players should be adept at faking an approach, for any matchup. You can't really approach, because it's too slow/predictable, so you have to pretend, and capitalize when your opponent reacts. Mindgames are your only option, of course. Your only reliable kill on Peach is your f-smash, but you'll have trouble getting it in. Never roll. You should probably avoid edge-hopping an f-air in general. Space with f-tilts, retreat constantly. Get in grabs whenever possible.
There's not really much to say about this matchup, because it's too stupid.
