CoolStoryBrobat
The hero Smogon needs, but not the one it deserves
I want to say "no" on this one, mainly because the hazards only activate upon switching-in. Losing an ability hasn't activated hazards as far as I know. If I had a Gengar, I'd go test this out myself but I highly doubt it would actually happen. You could theoretically test it by using Gastro Acid on a Flygon or some other Levitate abuser, so to speakWhen Gengar loses Levitate during Mega-Evolution and becomes grounded, does he absorb Toxic Spikes/gets damaged by Spikes/nerfed by Sticky Web on the same turn?