Fox is GnW's easiest top tier matchup... GnW can chaingrab Fox and Falco with dthrow from around 55% until they DI off the stage. Then you just fsmash.
Sheik is easily GnW's worst matchup. No need discussing that one
I feel that Marth vs. GnW isn't as bad as I once thought, but it's still like 65/35 in Marth's favor at the least. Marth is going to keep his distance most of the time, so you just have to approach at the right time. And then get grabbed lulz
Falco vs. GnW is just mean. GnW has ridic combos on Falco but Falco has lasers and dair + other stuff, no need discussing that one either
Captain Falcon vs. GnW is actually almost even IMO. If not then 60/40 Falcon. Falcon usually has around even matchups vs. most of the low-tier cast, and GnW is no exception. To me it's just a matter of who is playing better at the time, because GnW can counter or atleast trade on most of Falcon's approaches.
Peach vs. GnW... don't even try. It's bad.
Jigglypuff vs. GnW is actually a doable matchup, not in GnW's favor but it's a possible matchup at the least. Jiggs will just space her bairs but you can get a solid fair in if you time it right. And dthrow -> nair always works.
ICs vs. GnW... ugh. >_>;
And that's my short GnW matchup review, if you want me to go into more detail then... just tell me.