G&W... I dunno. Up until like, Vinnie level, G&W just sucks against MK. Dtilt, Ftilt, and GSL put together basically shut down all of his options.
Stay grounded as much as possible. Don't try to chase G&W into the air unless he's like right above you where you can just uair him. DO NOT OVERCOMMIT. Going for something greedy like a shuttle loop is begging for a dair.
You do not want to be above G&W. While you aren't helpless, per se, that uair is incredibly irritating. You're going to have trouble getting down. I recommend going far offstage-it's a better position, honestly.
Dtilt and GSL beat bair. Ftilt and GSL beat almost everything. Utilt beats key. Careful though, they can bait the utilt and punish you for it. Usually a well-timed SHUair or even just a shieldgrab/nair OOS is a safer option, but occasionally going for the utilt isn't bad. When you do GSL, get your *** back to the ground as soon as possible. You do NOT want to be stuck high up in the air at the mercy of G&W's uair while stuck in a glide (or worse, specialfall). Similarly, don't tornado. No, seriously, just don't do it. It's just really bad in this matchup. Like, "I will be punished on hit 99% of the time unless I do it absolutely flawlessly and even then about 50% of the time" bad.
Killing G&W... I've had a lot of success with Fsmash, especially as a ledge trap, but also just baiting. What hasn't worked so well is dsmash (G&W has very nasty punishes on block, and you don't want to get grabbed in this matchup). Glide attack... Remember what I said above; gliding is really dumb in this matchup. GSL is likely to be very stale... Keep bair and nair fresh, that's my advice. And don't forget that tipper utilt is a really good kill move-if they get too greedy with the dair, utilt will kill them around 120 (?) or so fresh. But just remember what you always have to remember here: don't get greedy. If you have to, just keep racking damage with tilts and GSL. Even fully staled GSL kills at some point.
...Yeah, that's basically all I know about the matchup, but it's pretty much enough. It's all about reaction time and playing stupidly safe and patient. This is a matchup you win by being reactionary and zoning perfectly. All there really is to it. Try not to lose the lead.
