Simple easy, pay attenion. First go get the yellow stickys and put them up all around the base of the plant, this traps the majority of the adults. Second and most important, go get a few potatoes from the store, peel them and cut little 2x4 inch pieces and place them on top of the soil at the base of the plant. The gnats can not resist them and will lay their larve on them. Change them out about three times a day durning light time and leave a set in over night. Even with an infestation in the hundreds they will all be dead in less than 48 hours. As you change out the potato pieces look at them under a scope you will vomit. lol
Never fails, works quick and no chemicals.