Kaiser is right on Bair. You have no excuses when it comes to Bair ****. It has so much range and priority, but any character can just SDI out of it and hit G&W with their fastest aerial (or if you're a ***** like Wario or Jiggly you Waft/Rest him out of it T_T).
Bair will eat your projectiles and stop approaches very well, but as far as aggressive G&W goes Nair tends to be the thing to fear. That will pile on 17%, and then link into another Nair.
G&W ***** his recovery as soon as you have to use PK Thunder with the Bucket, or an even more hilarious Uair/Up B to screw you over with the wind box.
He is also going to have trouble killing you as long as you are smart. Smashes are hard to land, so just keep your spacing and avoid rushing him down. Often times if you see G&W charging a smash it is just a good idea to wait it out instead of trying to punish the smash. G&W can easily abuse the charge time of his smashes along with their priority, surprising speed, and in Fsmash's case the lingering hitbox to trick you into a smash. If you avoid the smashes he basically has to kill with Fair which will take a while (towards the 150% range on stage). But once he gets you offstage, that's where he will get kills.
Honestly it's all about hoping you can find a way inside his priority. He'll beat you down while being aggressive, and he'll beat you down while being defensive. He easily deals with your projectiles, and your recovery (thankfully you have something else to work with because of Zap Jump, but it's still a grim tale).
Granted, Ness will deal heavy punishment if G&W screws up, and G&W can actually be edgegaurded now thanks to NASL. Technically G&W could sweetspot the ledge, but it is basically never worth it because his Up B goes so **** high that he has to go stupidly low to sweetspot. Generally speaking G&W is best off recovering high and landing on the stage, or just using his Up B early on and then going for the ledge while defending himself with Nair, Chef, Fair or whatever.