Initially, it seems like G&W owns Wario hard, but once you learn a few things about the match-up, it's not that bad.
Dr. Mario Guy and Gea have made good points. G&W's Bair and Fair are all pretty easy to avoid and punish if he lands, but you do NOT want to attempt to punish from the ground in general since the move will likely be spaced well and you'll eat a Dtilt if you attempt to run in or shield grab. Use things like Dair and Chomp from the air to punish his landing lag. If you get caught in the Bair, you can smash DI up and get out of it before the strong hit box appears.
Wario and G&W are fairly evenly matched in the KO percentage department, but Wario's KO moves are much faster. As mentioned before, avoid G&W's smashes at all costs. Don't attempt to punish a missed Fsmash in general since it has very little cool down lag. Tech the Dthrow (press L/R/Shield button right after the 3rd "beep" of the juggle) to avoid Dsmash. IMO, one of the best things G&W has in this match-up is if he guesses your tech correctly and lands a Usmash since it is crazy strong.
In general, play "hard to get." With experience, you'll learn when openings really appear and when his moves give the illusion of openings with their crazy hit boxes. When openings are there, G&W can take 20-40% easily from a couple of Dairs/Uairs/Chomps. Once he gets to 80-90%, a "Half-Waft" should take him out. Play it a little more conservatively on using the Fsmash to KO (probably 90-100% to be safe) since he cannot be edgeguarded safely in general.
Honestly, I see this as maybe 40-60 in favor of G&W; but the more I learn the match-up, the more even it seems. IMO, it's a little easier than Meta Knight and about on par with R.O.B.