Whatever you do, so NOT put fly paper/tape IN the gecko tank, that could lead to some potential serious issues (I would not want to try and remove a gecko from that stuff!)! I've heard of people making fly traps using monster energy drink. Try searching gnats, I seem to remember some old threads with people talking about various ways to eliminate them. Good luck!
Help! Crested Geckos! 0.1.Leachie, 2.1.1 Gargs, 1.0 Chahoua, 2.1. Blue Tongue Skinks, 1.1. White Lined Geckos, 1.3. African Fat Tails, 2.2.0 Tokay Geckos, 1.0. Giant Day Gecko, 2 tarantulas