Well, I don't go on these boards often, and I only play online, so I guess that invalidates my opinion haha... but I'll say what I feel I have to anyway, at least in respect to the online aspect of this game.
G&W is a character that is, in general, weak against any character with a projectile at the core of their strategy. G&W is all about ensuring your opponent is under pressure to move in a certain direction or do something specific so that you can hit them before they realize what you are doing. Without a reliable projectile of his own, G&W cannot reliably play mindgames with someone who has projectiles because he will constantly be forced into traps. Namely, some of these characters are Olimar, Snake, Diddy, and to somewhat lesser extent, ROB.
Olimar seems to have the upper hand when fighting G&W. He will throw pikmin at you from a distance if you do not attack him, and when you do attack him, he plays the whole grab / smash jig and ensures you cannot get close to him with a turtle. Keying is somewhat reliable, as I've found that he cannot usmash it to counter, but after a while it can lead to him predicting it and grabbing you afterward.
Snake is ridiculous and is definitely a G&W counter imo. He can play with many projectiles on the screen at once and set you up for some attack in case you don't get hurt. He can also edgeguard you like a monster and make sure you never get back down on the ground. Not to mention that his ftilt and utilt are GREAT for dealing with turtle and key respectively, no matter how well G&W spaces them. In addition, if you fall back and play defensively, he snakedashes or grenade camps and BOOM, you're up in the air and ready for another round of edgeguarding (if you didn't die from the attack already)
Diddy is incredibly annoying. Normally, he is extremely weak against G&W and has nothing on him. However, once he gets those bananas in play, you cannot move and approach him reliably. You will have to hold back and play uberdefensively (something G&W isn't good at should I mention) while he sets you up for a popgun, a smash, or a dash combo.
ROB, as I said, isn't TOO hard. You can easily rack up damage on him with turtle and fishbowl. The problem? You just can't kill him reliably. Once he is high up in damage, he starts camping with laser and gyro in addition to dsmash. If you approach him with a smash, you're dsmashed before you can get into range. If you turtle or key him off and edgeguard with something, good luck, because it will most likely not work because of his insane recovery / edge camp. If you fair while he's on the ground, you're grabbed. Mindgames that do not exactly ensure victory are a major part of dealing with ROB.
Well, that's all I have to say haha. Those are the most likely characters to beat G&W in my opinion.
PS: I personally don't find Toon Link to be too hard. His projectiles can easily be seen before they are used and often don't lead into setups; in addition, his killing methods are easily predicted and countered.