i heard soil drench and hot shot strips works good. i think you can use them mesquito dunks
and theres also a nematode that kills them
and your hot shots will kill off the flying ones
the dunks and nematodes are non pesticide, cant say the same for the hot shots, from what i hear they are some nasty chemicals that are released from these things but they kill the shit out of any and all bugs in the area. if you decide to use them use cautuion and follow the labeling. also by no means is it a good idea to use them in a grow room that is maybe in a bedroom closet. especially at night.
good luck with your gnat issue, i have had to deal with them before and they are a PITA to erradicate.