They are attracted to red wine so a glass of that in your room will work too.. Like already said, Bacillus Thuringiensis works very well. You can also use a topping such as sand, perlite, DE Powder or "gnat nix" (It dries out fast and they don't like to lay eggs in dry areas & I believe gnat nix is suppose to cut the protective layer on them killing them)
Overall, most effective is BT (Gnatrol or Mosquito Bits, etc) I have to deal with them a lot and this usually gets rid of them within a day or two.. Anymore I just use it as a preventive.
On a sidenote: You can sprinkle DE Powder around your grow room to.. When the gnats step in it generally kills them. Fun to watch them roll around in it suffering a slow, painful death trying to escape.
Best of luck, my friend.
