Oh boy, this can be a fun MU, I play 2Fast in it a bunch.
First and foremost, all of our aerials will beat fireballs, including the windbox for uair. Luckily for you guys, fireball up close is a bad idea for us at close to mid range to bucket, so if for some reason we decide to suck up the fireballs, you can run in and punish. However, if we do manage to fill the bucket up, the damage on hit is 33% and it kills pretty well, so be on the look out. An aggro mario will get dominated in this matchup, so you should look for openings rather than rushing in and attacking. Cape has zero effect on our recovery, don't waste your time (the first time 2Fast did this to me...Looooool) one cape won't do anything nor will three. Don't go offstage and spike too low, as UpB will beat it every time. Despite being lighter than Mario, with bucket braking and DI, we outlive Mairo, so keep that in mind and keep your kill moves fresh.
Not sure how you guys do with platforms, but G&W excels with platforms, as being under an opponent can wrack up some mean damage as well as uair shenanigans. Good stages for us include Frigate, Battlefield, Rainbow Cruise, and Brinstar. Decent to bad stages are Final Destination and Yoshi's island.
This matchup actually isn't too bad for Mario, once he learns the match-up, it becomes much more even, though, still in Game and Watch's favor.
Quick note, even though G&W's don't utilize it enough (in my opinion) watch out for Chef off stage, It WILL gimp you.