I can't stand ZSS. I know that you can bucket her projectile which she relies on for BnB combos, but everything else seems like a pain in the bottom when fighting her. Side B can easily deny our Dash Attack approaches and Dash Grab approaches, and I find that you rarely have an option to punish her Side B out of shield unless you're in close range and you perfect-shielded. I haven't been able to find a reliable way to deal with Down B either. We don't have a regular U-Air, I've never been able to beat it with Nair, and I never find myself in a position to use Fair or Bair. Dair is also ineffective for obvious reasons. Up B is our best bet but you will trade at times and Down B is not a pretty move to trade with. Down B is also safe on shield which is really annoying, you can't even chase after her with a Dash Attack. If you do have a full bucket, ZSS will just go spam-happy with Neutral B because she knows you don't want to risk throwing out an Oil Panic, and she knows that she can play it super safe while you're just kind of a sitting duck as a result.
G&W is a ******. Not here to make the matchup sound better, but you can literally duck under a bunch load of things ZSS does.
What you can duck under:
Dash Attack (wow rofl)
Grab (don't know about side B)
Lasers
Fsmash (need fast reflexes)
Jab (I think)
Those who don't know the matchup would panic and start to predictably use their dtilt and dsmash. I believe you can beat dtilt with your dash attack. Dsmash leads to bucket. I wouldn't bucket lasers since you're most likely getting dash attacked in the face. And those who know how to torture well can allow you to bucket up to 2/3 and never shoot lasers again.
As for her down B, shielding can be an option, but you're doing what she's expecting you to do. Maybe you can challenge her down B with usmash? I did that with Pikachu's usmash and ended up killing her. But be wary since you get buried for waaaay too long.
Remember I'm speaking as of now. Who knows if one of the ZSS mains discovers this/reads this post, only to become aware and counterplan maybe.
And if you really want to know, I'm pretty sure your dash attack > hers. Half the time your dash attack hitbox is already out but she hasn't started hers yet.