This MU is hella hard, but it's actually winnable. The key is spacing. Marth will zone the **** out of you if you let him. You have to be able to quickly transition from well out of range to right in his face, as that is really the only way to play it. G&W effectively zones a lot of mid and low tiers, and so a lot of players have this playstyle of keeping people right at arm's length the whole time. Marth gives G&W this same treatment, and if the G&W player tries to play it the same way as the MUs where G&W does the zoning instead of getting zoned, they will lose. You have to play a hit and run, dodge and weave kind of game. Bair and nair are invaluable, as well as fair, upb, and dtilt. You should also grab a lot, as Marth players are really just looking for you to do something on their shield so they can upb. A lot of players have it in their head to challenge Marth's fair with our own. Most of the time this ends badly. Let Marth jump around and try to fair you, then go in when he's trying to land, as he really doesn't have that many safe anti-air landing options.
This MU can be really ****ty, but it can also be very very winnable. It's all dependent on both the Marth and the G&W. -1 or -2