Firstoff, Marth is not that bad if you know what you're doing. You can trade hits with most of his stuff with F-airs, and you outdamage him in this situation. F-air also gets him offstage really easily, which is even better. Learn to SDI DB and you should take little damage from it. You can also potentially get a free D-air or B-air from it. Edgeguarding and juggling of course works extremely well.
Diddy Kong...is really annoying if they're good, but the matchup is fine if you're good at Z catching Bananas on reaction, and occasionally swings to G&W's favor on counterpicks.
Vs Donkey Kong, I just think G&W players don't play this matchup enough. G&W has the tools to shut down Donkey Kong really solidly. DK is easily shield poked, and N-air out of shield punishes almost everything he does (assuming that he usually approaches with B-air and other aerial mixups). He's also very easy to edgeguard, and once you learn to bait the <100% getup attack by learning to do turnaround grabs out of shield, it's not that bad. On BF, you **** him when it comes to edgeguarding. D-smash, and especially D-tilt hit below the platforms, giving you the perfect stage control options for edgeguarding him. Learn to not get baited by airdodge -> U-tilt mixups and stay patient when he does Smashes.
Olimar similarly, I just think most G&W players aren't familiar enough in this matchup. It's really a very simple matchup for G&W. Just outpatience him with D-tilt and N-air until your N-air hits him, and then **** him. D-throw also ***** him a lot even if he techs. Generally speaking all you need to do is take advantage of the fact that Olimar should really never be approaching and stage control him to death.
Vs Lucario is also in G&W's favor...don't get baited by forward rolls, F-smashes, and full charged Aura spheres and there isn't much else he can really do.
Vs Sonic....he can't kill you period if he doesn't bait a dodge, so don't dodge unnecessarily and you'll win.
Ike can't do anything if you powershield his aerials on reaction and just outspace his Jabs, and like Sonic, he really can't kill you without baiting a mistake.
That's all I really care to talk about, and there are many numbers I also disagree with, but arguing number discrepancies on that level would be pointless.