I would advise against putting a layer of DE on top of your soil, although sand might help. DE needs to be dry in order to work, and the particles are also very fine; this can lead to a sloppy mess. Dry DE spread around the floor, in crevices or wherever else you see them will help to kill the adults. Once they come in contact with DE they will die within 48 hours.
No pests strips? No where near my plants.
So, the root of the issue here is that indoors in your grow room fungus gnats have [virtually] unlimited food and no predators. The result is the potential for their populations to really get out of hand, in a relatively short period of time and before you even know it. The fungus gnat life cycle is about 4 weeks and there are 4 stages to this life cycle (egg, larval, pupal, adult). The adults are the ones which are most evident, and so they're the most often targeted by gardeners. But if you have an infestation you have to take multiple steps and target the fungus gnats while in larval and/or pupal stages as well.
Don't under-estimate the power of a good vacuum against adult fungus gnats. Vacuum your grow room while you water to catch the most adults. Do this as often as you can if you have an infestation. Here are some other safe recommendations I have given in other gnat threads.
Pyrethrin as a botanically-derived insecticide that kills on contact and breaks down within several hours, it is not toxic to birds, mammals or amphibians and leaves no toxic residues which is ideal for our purposes. The spray is widely available at most home improvement centers. I am pretty sure this is the same stuff as Dr. Doom, only that brand costs more than it should. This is also good for killing adults on the soil surface.
Mosquito Dunks or Bacillus thuringiensis isreali (Bti) is a natural bacterium that produces a larvacide which is toxic only to a handful of species of insect larvae including fungus gnats and mosquitoes. It is widely available in compressed circular pucks under the brand name Mosquito Dunks, sold in most home improvement centers. They also sell Mosquito Bits, which is the same thing and another brand is Gnatrol, but these products are usually less available. If you get dunks you use them by soaking in a bucket of water over night and irrigating. This should kill most of the larvae; although it is best used to prevent a gnat population from taking hold in the first place. If you already are infested this is still a very necessary step in order to kill the larval stages of the gnats. Re-apply every two weeks even when you don't see adults.
Then of course you should have yellow sticky traps around and Safer brand or Gnat-Stix sticky traps in the containers. But it is hard to bring absolute elimination if you have a lot of plants, house plants, etc. Biological controls are really the best. Someone mentioned Hypoaspis mites, which predate on the pupal stages of various insects including fungus gnats. I have these myself and they do quite a job. Given a sufficient food source (like gnat pupae) the Hypoaspis mite population can really boom; once they've eaten the majority of the pest insects their populations may drop although they can subsist on dead organic matter/vegetation.
Hypoaspis, Bti and beneficial nematodes are a sure fire way to rid your garden of gnats and keep them form coming back.