Comes in the soil/ you bring it into the grow room. THere are a million ways. Do what mauh12 said, or for a real sure fire way get a sulfure burner However that doesnt fix soil problems.
somebody here told me to put play sand on top of the soil . it traps them , they cant get in or out. i did it and i now have no more gnats. hope that helps
#1Get some hot shots pest strips and put one in your grow room.#2get some neem oil and spray down your grow area. always make sure to clean you grow area thorouly(important). good luck