Ive been battling these flying fuckers for a long time, and didnt even know the larvae ate the roots, they are pure evil. Gognats works great but you have to drench everything at once, cuz the adults just fly off and lay more eggs after the go gnats dries up. I use a 3 step approach, drench with go gnats, you will see all the youngins crawl to the surface and freak out and jump everywhere, then hit them with a pesticide spray called doktor doom it will kill on contact as opposed to the gognats which just seems to make the soil highly uncomfortable for a while, and then use those yellow sticky fly traps so the adults dont fly back and lay more eggs ( only a buck for five). On a side note, only use doctor doom while in veg, dont want that stuff on your nugs! I was also told to let the soil dry out completely, as wet soggy soil is exactly what fungus gnats favor, so instead of drenching all the time, drench once and spray the surface until the whole container dries out again, and then reapply. I used neem oil, killed spidey mites, not the damn gnats! I tried predatory nematodes, I thought it worked for a little bit, but maybe it was just a placebo affect, has anyone tried these before, maybe I just didnt apply them right?
AFTER your seedlings are about 2" tall, you can put a 1/2 -1" layer of clean playsand on the top layer of the pot. The gnats have larvae in the soil and they won't burrow out of the sand as it hurts thier tender little bodies. Basically this breaks the life cycle of the gnats and they will die off. No sprays, no nasty chemicals needed. 
