Ok... typical situation here. MK knocks Pit off, Pit has all of his jumps. Say he just got dsmashed. Pit DI's properly, and MK runs to the edge.
Pit has two options:
-He can fly high back over the stage and fall back down snake style. MK will probably wait for the airdodge, so Pit can wait for MK's attack and as long as you're fast enough you'll be safe-- tornado aside, though you can SDI out of that and at the very least get yourself to the ground, if not punish MK for doing it. In G&W's case, though, you can't really airdodge his fair because it lasts longer than your airdodge, and unlike the tornado you can't SDI and it will probably send you back off the stage. You have to use your airjumps to avoid getting hit, and if he wants to he can just wait for you to run out of those before he throws his fair out.
-He can initiate a glide and attempt to go under the stage. If you dive very low (I mean, very, very low, to the point where you're not on the screen anymore) MK cannot reach you, unless he's down there already waiting for you. Even the ridiculous shuttle loop doesn't have the vertical distance to deal with this. He'll probably kill himself if he does hit you with it. If he runs to the opposite side to try to head you off, you can pull up and grab the near edge anyway, and even if you mess this up and he's in a position where he can edgeguard your glide (should be rare), you can cancel out of the glide and use your airjumps/up-B to throw him off. It's certainly not your best option, but it is possible.
G&W, though? If he drops down with a fair or bair, then up-B's... it's probably going to hit you. There isn't really anything you can do about it. Even if you attempt to abort the glide he can cancel his up-B on the edge then do it again to kill you. If you do manage to go under it-- really more his mistake than anything you could have done-- the trampoline from the up-B can throw you up above the stage with him, in which case you're vulnerable to the same BS as if you recovered high above the stage.
Just saying... I very rarely ever get gimped by MK myself. It's just something you have to adapt to. G&W however... to this day I still can't find a way around that crap. -.-