Make sure you've got them identified as fungus gnats rather than thrips. If you are seeing damage to your plants, thrips are the more likely culprit, as fungus gnats aren't going to chomp on your plants. Thrips are smaller and it is the larvae that cause damage. Fungus gnats are a slightly larger, plumper, bug with larger wings relative to their body size, and can only live and breed in a grow area that has stagnant water or soggy soil in it somewhere- floor, overwatered pots, the runoff trays beneath them... which you shouldn't have a anyway. Remove the excess water from your grow and any fungus gnat problem will go with it. Thrips require a more concerted effort to remove; one simple trick is to cut a slit in a paper plate and slip it over the top of the pot, preventing the larvae from dropping to the soil to pupate and emerge into sexual adults.
Good luck.