Out of all my matchups as D3, these two give me the most problems. I can put up quite a fight against my bud's MK (Who's decent) but a really good MK gives me trouble. My main problem with G&W is the fishbowl, Uair, and the turtle. I'm decent at outspacing a fairly good G&W with Dairs, Bairs, Ftilts, and Dtilts. I'm in trouble when G&W hits me with the fishbowl - once he gets me with that I gain an upwards of 2903582903 damage. Another problem is the uair. The only time I can stop this is if he screws up, if he spaces poorly and I get him with the dair, or if he's actually feinting and he traps me by fastfalling into an Usmash or something. :/ Turtle isn't too much of a problem tho. I'm slowly getting the hang of shield-grabbing G&Ws who space this move poorly, and learning to escape those who can space properly. Also, the G&W's I face are slippery. >_< I'm having trouble trying to get at them even with my spacing moves.
My main problem with MK is getting some breathing room. I can hold him off decently with bairs, inhales, fairs, and dairs. I've also really gotten the hang of stopping his upb when I'm above him with the dair. However, once he gets past my comfort zone, I'm usually done for unless I can swat him away with a lucky bair or dair. The nado is a big problem. All the good mks I face are smart and they use it when I least expect it. I also hate, HAAAAETE his dsmash. He spams the everloving **** out of spotdodge-dsmash-EDGEGUARD on me and it drives me nuts.
SH Fairs are really annoying, his superfast tilts are annoying, the spotdodging is annoying - everything is annoying. Also, I'm helpless when MK is edgeguardin me. I can swat him with the fair sometimes, but it always boils down to him dairing the crap out me untill I'm forced to use UpB, upon which he continues the dair me, or he gets in real close with an airdodge and gets me with a reverse upB.
Don't say pick a different character. I would usually use G&W against MK, and Donkey Kong against both, but I'm looking at really improving my D3 game (Whom I want to main alongside DK and G&W). I figure that if I can beat these two with Dedede, I can beat (almost) anyone with Dedede. 
My main problem with MK is getting some breathing room. I can hold him off decently with bairs, inhales, fairs, and dairs. I've also really gotten the hang of stopping his upb when I'm above him with the dair. However, once he gets past my comfort zone, I'm usually done for unless I can swat him away with a lucky bair or dair. The nado is a big problem. All the good mks I face are smart and they use it when I least expect it. I also hate, HAAAAETE his dsmash. He spams the everloving **** out of spotdodge-dsmash-EDGEGUARD on me and it drives me nuts.
SH Fairs are really annoying, his superfast tilts are annoying, the spotdodging is annoying - everything is annoying. Also, I'm helpless when MK is edgeguardin me. I can swat him with the fair sometimes, but it always boils down to him dairing the crap out me untill I'm forced to use UpB, upon which he continues the dair me, or he gets in real close with an airdodge and gets me with a reverse upB.Don't say pick a different character.
I would usually use G&W against MK, and Donkey Kong against both, but I'm looking at really improving my D3 game (Whom I want to main alongside DK and G&W). I figure that if I can beat these two with Dedede, I can beat (almost) anyone with Dedede. 