Mosquito dunks are your friend, just crumble or better yet, rasp a few kernels into soil or/and water. Always neem and DE
Just make sure the wings on your critters lay flat or you may have root aphids, winged. I had fg's for months on end. Repeat the dunk treatment every 2 or3 wks. Drainage in soil mixes for LEDs is a big deal. The above treatment will free you from spider mites also. I put dunk crumbles in all the house plants and all my mixes and neem about every 3 wks when no pests are present.
One "gnat" could even be a fruit fly, real concern sets in when they look like tiny flocks of migrating black birds.