Having a REALLY hard time against

... Any mu tips?
I'm rusty on this MU, so everything I say here might not be 100% correct, but should hopefully give you a basic idea.
Neutral
G&W is a character that wants the opponent to approach like Zelda, so neutral is largely going to be about baiting the other into approaching. Neither character has good approach tools, but Zelda without Phantom is worse. So you need to up your mind games and pressure him to approach while not taking any baits to do so yourself. You can do so by mixing up full charge Phantoms to bait Bucket(reflector) then punishing it and using your movement to confuse and invite approaches.
Main thing to watch for is Bair. It's a heavily disjointed multi-hit, so is a pretty good spacing tool. Thankfully it's one of his very few, but it's not easy to work around either. Pay close attention to his spacing. If he spaces into shield, it's not safe so you can punish it. If he spaces away it's pretty safe. Try to avoid situations where he can spam it freely, by either sticking close to him and ensuring he has to give up stage control to use it or making to stay directly below him if he is in the air.
Once you get that figured out, his other aerials aren't as effective. Nair is ok, but isn't particularly scary. Dair I don't think is safe on landing, so spacing an Fsmash outside the hitbox range should do it. I think parrying the landing hit might also work. Fair cross-ups are pretty decent, so don't drop shield too early if he goes for that. You can parry Fair pretty easily.
He doesn't have many spacing or neutral tools on the ground. His projectile is annoying, but can be reflected. Dsmash can be used to stuff approaches and is actually pretty scary, doing lots of knockback or even grounding you for a ridiculously powerful for no reason Fsmash follow-up that can kill you at 80. Be very careful when in its range.
The thing you need to be careful of on the ground, is F-Tilt. It's fairly disjointed with decent range and frame 8 startup is nothing to sneeze at. It's safe on shield properly spaced, so don't try to punish it or you're eating another one. It can be punished on whiff with Fsmash otherwise.
Main thing is to not approach in this MU. What little G&W has in terms of forcing approaches, Zelda can counter pretty easily. Mixing up with the full charge of Phantom will force to approach, just be careful of his reflector. Don't think you need to start approaching or attacking if there's a lull in the battle, wait for him to do so instead. One of the most common mistakes I see Zelda players make, is that they crack and try to approach, usually with dash attack. That rarely ever works and will be a free punish.
Advantage
There isn't much to say here, G&W has a decent disadvantage, so go for Up-Tilt and Nair strings at low percents as usual. You're going to have to focus more on catching his landing, as he's too difficult to chase or press advantage and attempting to edgeguard his amazing recovery is likely going to get you stage spiked. Focus on ledge trapping and the occasional Up-air snipe when he gets too comfortable hanging around in the air. Be careful of drop down bucket when going for Phantom ledge traps. I'm not sure I've ever seen it work well, but it might be a thing.
He dies to pretty much anything after 100.
Disadvantage
G&W would normally have some scary combos, particularly with that ridiculous Up-air, but Zelda has good tools to escape. Up-air is a projectile, so you can reflect it with Nayru. Nair is decently disjointed, but is relatively long lasting, so depending on spacing you may be able to Up-B into it. I think Fair can also be reflected, although I don't know how effective it is to do so. Bair and Dair won't give him much.
He doesn't have much to work with at higher percents, so don't worry about the combos so much and focus on the kill options. He has powerful but difficult to hit kill options. Dsmash -> Fsmash is a good confirm, so be wary of Dsmash. Dsmash without the ground is decent but won't kill till at leas 120(I think). Depending on where you are at, it will either send you straight up or at a low diagonal angle or ground you. I think if he hits you close it will ground, if he hits you far up the mallet, you will be sent diagonal. I think if you are in the air, it will send you straight up. DI/mash accordingly.
Other than that, he basically has an invincible Mario like Up-Smash, so don't try to land into him ever, Fair which is a decent kill option when unstaled at around 130ish, Bair and F-Tilt at high percents(150), and Dair at really high percents(?). Not sure about that last one, you shouldn't ever get hit with it anyways. Just be cautious and don't panic and do something stupid and he'll have a difficult time closing out the stock. As your percent climbs, so does your rage and you can kill him pretty early if he can't close your stock. Every kill option outside Dsmash has short range and it's unlikely he'll ever be able to catch your landing with them, provided you don't make it obvious of course.
Offstage, Zelda is still impossible to edgeguard, but G&W is one of the few caveats. Farore's Wind has vulnerable moments and can be punished with sufficiently long lasting aerials, such as Palutena Nair or G&W Bair. That won't be too much of a problem if you practice your teching, but it's something you need to be ready for if he decides to suddenly drop down.
Fair and Dair done from high above the stage can 2-frame, but it's not easy to do so. Be ready to tech there, although I'm not sure if they stage spike.
Patience, patience, patience, when dealing with other campy characters as Zelda. In the vast majority of cases, Zelda should never have to approach. If you find yourself in such a situation, something went really wrong somewhere and you'll need to correct that. Even if it takes a long time to wade through spam, they'll screw up eventually and there's not point in taking unnecessary damage trying to force an approach. Pay close attention to stage control, how your opponent is spacing aerials, and try to position yourself in a way that makes it difficult for them to maneuver. You've been blessed with a myriad of good OoS options and grabs being weak in this game, so abuse that shield as much as possible.