Hey, I'm a little late to this thread, I main both G&W and Zelda so I'd like to think know both of their options pretty well. That said, I'm pretty new to these forums, so I'll default to whatever more experienced people have to say.
Din's
ABUSE your fire buddy. But do it a little differently than normal. Yes he can absorb it, but it's WAY easier for him to do that when it the ball is moving. Once it's stationary he has a lot less options. He can work around it or confront it, and both should give you a good read. If he confronts it he can use a disjoint on it the way a Swordsman would, or try and absorb it.
To absorb it he would need to wavedash in or drop into on a jump, and neither are great options because the Bucket has braking properties. He will also be stuck in a bit of end lag if you want to approach during this, especially considering he will need to drift in slowly under the brake. It's especially difficult for him to absorb if you place it under a platform. So this would encourage him to use that platform, which is often times a good height for your Up-tilt or Up-smash.
If he knocks your fire buddy away with a disjoint, depending on the move he uses he will be stuck in some endlag you can capitalize on. His moves generally have a lot of duration, allowing you to approach. Based on where you place the Din's though, you can make him clank it with more laggy options. Put it just high enough so he can't jab it. If he wants to hit it his options are U-tilt, Side-B, or a short-hop aerial. These all make him predictable and punishable.
Killing G&W
Most of your standard kill options should work, but you need to use them situationally. He is super light, so he dies quickly off the top. Your Up-air out-disjoints his Key, and your Up-tilt is very dangerous for him if you can get a hard read. His horizontal recovery is also not particularly good. If you can penetrate his wall of disjoint your kicks will wreck him, and your back throw can kill early too, especially if you catch him with bad Di. F-smash is good too, but I think the kicks and Up-tilt are better options.
Working against his disjoints
Punishing unsafe horizontal approaches with Nayru's works as normal, and it's something G&W has to respect. The Key will be dangerous though, because it will penetrate your Nayru's from above, and with it's landing hitbox, spot dodging is not a good option either. You just can't allow him to be directly above you. In the air, use your B-move mix-up options to get down from juggles. Your D-air stomp will not out-disjoint his Up-air or Up-tilt, and it will be very difficult to time on his Up-B. For his grounded disjoints, its a game of calculated spacing. Keep your distance, and take advantage of of his long hitbox durations and lagtimes to set traps. Catch him doing something stupid, and punish.
G&W being faster than Zelda shouldn't be a problem on it's own. Most of the cast is faster than Zelda; she shouldn't have to change her playstyle too much there.
Maps
As a Zelda, I would try to play G&W on PS2. Lower ceiling, just the right amount of platforms, and a big flat open space to get away from him. FD is pretty good for you, again with the space. I think GHZ is pretty neutral, you can both kill each other easily, and the flat space is good, but the platform doesn't work in Zelda's favor in my opinion. Smashville leans slightly in Zelda's favor IMO. You can camp well on Norfair, Distant Planet has a lot of space too.
If I was the G&W I would want to take you to Warioware or either Yoshi's. Small stages, platforms, and mobility are good for him in this matchup.