I dunno about fungus gnats, but it sure did a number on the aphid infestation I had in previous years of indoor growing. I had tried all the hippy dippy crap and quite frankly the 'remedies' did more damage to my plants than the cures. Two applications of Sevin and the aphids were all but done. Of course you do have to take the appropriate cautions (don't let your pets get at it, don't spray it in your child's face, don't drink it etc.). Hope it works for your gnat problem, although that may be a bit different considering they tend to lay eggs in your soil.
Neil