Dair is simple if the G&W gets predictable (G&W LOVES to Dair you while they're above you, whether you're grounded or not). If you're on the ground, just shield the Dair (It has a hitbox upon landing tho so watch out your timing). Once G&W lands shield-grab, pivot-grab, or do some other really quick punisher, as G&W can pull out another attack almost instantly. However, note that this method isn't fool-proof as the Dair hits through shields sometimes (I dunno how. Whenever I face G&W the dair goes completely through my shield sometimes. Also, just an interesting thing to note but sometimes I can shield through the entire duration of Bair, even with the landing hitbox. Brawl is weird like that). Alternatively, you could just run away from it or roll away from it (keyword: AWAY).
However, if the G&W is smart he will use the move in a situation where he knows you will be caught with your pants off and you won't be able to punish him. While you're in the air, while you're on a platform, keystomping, even off the edge if he can still make it back with UpB. There's not some definite counter in this situation, because in this case, the G&W gets you into this situation and/or is outplaying you and you need to try and outplay the G&W. While you're in the air, your best bet is to DI away or air dodge. Also, something I find that really works like a charm is fastfalling into a roll or spotdodge (Assuming your character of choice as a quick fast-fall - someone like Luigi will fail in this situation). You gotta have killer reaction time for this tho, and the G&W has to at least be a fair distance away. Don't try and attack through the Dair unless you have ungodly timing, it has totally BS priority.
Once thing to note about the Dair is that G&W telegraphs it a lot (Although its speed is still killer). If G&W is right on top of you and he dairs (Honestly he should be using nair or something but it'll happen) you can try to smack him out of it with a quick, high-priority aerial.
However, if G&W is just out of your attacking range, but still too close for comfort and he dairs, the only thing you can really do is air dodge and jump away, fastfall and roll the hell out of there, or DI away (If you're Wario that is lol).
Another trick to watch out for is slowfalled Dairs. If you have a twitch, uncontrollable reaction and the G&W knows this, the slowfalled Dair can screw you over if he gets you.
BTW, if you're G&W as well use uair. It's a great opportunity to refresh your moves and possibly trap the other guy into a Usmash or UpB.
That's the only advice I can think of from my matches against G&W (BTW I use Marth or Donkey Kong against G&W). :/