I'm not a G&W, but Lucario and G&W are probably the matchups I know best with Peach, and I've been coaching a Lucario against a G&W so if you don't mind, I'll join in here. I've also played with Xefnir, Kurohito, phi1ny3, and Samuelson IRL from the Lucario side, and Valdens, Onski, Corimon, and a few other G&W's in NorCal.
Definitely G&W's favor, for starters, but it's a very interesting matchup. G&W doesn't really have much ability to punish Lucario's fsmash out of powershield, unlike most characters. While G&W's bucket prevents aura sphere camping, it doesn't prevent the Lucario from throwing out charged spheres at close range since bucketing them results in a free fsmash for Lucario.
What the Lucario's I've watched usually do is camp G&W's with retreating rolls, fsmash, and ftilts, then using jab to punish spacing mistakes. G&W's nair messes Lucario up in the air and the Lucario has to be wary not to push his combos too hard because of G&W's up-B (and don't dair his shield >_< ).
Lucario's jab, ftilt, and fsmash seem to be the best moves in this matchup. Due to Lucario's superior range, the G&W is the one forced to approach in this matchup, but he has all the tools necessary to do so. Problem is that the fact that Lucario has to rely on spacing fsmash in this matchup means it will probably go stale and with bucket breaking G&W will not die as fast as one would hope.
Most importantly, low percentage kills screw Lucario over. If G&W lands a smash attack at 80, Lucario is probably not going to win the match, despite his ability for comebacks.
I've seen G&W vs Lucario matchups run the timer out- it can get pretty gay, since Lucario shouldn't be approaching :/
I'd have a tough time rating this matchup with a number, as I'm the coach and not the player here xD But I'm thinking 60:40 in G&W's favor. You guys decide. Hope my input helps
EDIT: Also, I would also like to note that Peach vs G&W is my absolute favorite (in terms of fun) matchup in the game. If any G&W's wish to MM me at Genesis, I'll take it. It's a hard matchup but I just enjoy it too much xD
Lee Puff, you're mine.