diatomaceous earth Or neem oil?

It has already been mentioned but BTi is excellent preventive and will kill the larvae in the soil. Although their is other options as well, such as cedar oil which is the primary ingredient in gognats but just go light on the product and run it about 1/2 strength on whats on the back of the bottle at first. You can also add a 1/4"-1/2" layer of sand on top of the soil which will dry out quickly and prevent the adult gnats from being able to successfully lay eggs and continue the cycle. Usually it is poor moisture control is what leads to a gnat problem because they prefer damp soil in which to lay their egg so by controlling the period of time in which your soil is damp you can greatly reduce their numbers. Also if you have other indoor plants in the house be sure to monitor and treat them otherwise they just provide a vector to your grow.

I agree with what Polyphemus said, I have seen good results with BTi however I use beneficial nematodes and they kill the gnats outright in a weeks time. No need to traps or anything. similar killing process to BTi=bacteria attacking larva stage, nematodes=microscopic parasitic worms attacking eggs and larva. One note about DE, is that it is a respiratory irritant and if your sprinkling it on plants or the medium then pointing a fan at it, its asking for issues. Plus if its in flower it will stick to the buds and you don't want to be smoking it. Stay away from DE, add it into the medium for calcium but not as a pest control. Also might gunk up your tent filters should you use any. Good Luck!

Bti -- Bacillus thuringiensis subspecies israelensis -- ...common brand is "mosquito bits" or "mosquito dunks" -- bits are a top dress, dunks go in a reservoir. The dunks defnitely leave chunks of material in rez. you can use this preemptively. it is a subspecies of bacteria we have used on the world's food crops for a century? more? a long time at this point. it is safe. top dress every 2-3 weeks and you will rarely see anything that comes from larva in soil -- not just gnats. You may see 1 fly around occasionally but it will never escalate beyond that. mid-problem they will take 2-3 weeks to have full effect. but continue sticky trap use to kill from both ends of life cycle to speed that process up. nowadays i use bti the first few weeks after sprouting and after any transplant - whether i see anything or not. These pests often hitch a ride from the store in your substrate. If i don't see anything after a few weeks i stop using Bti. I slap a yellow sticky trap down in tent somehwere and maybe 1-3 flies will be caught in an entire grow cycle at worst. i don't even bother to do this too often anymore. DE will impact insects, but they have to walk through it. I do not suggest putting DE on top of your substrate. This will lead to a gooey, water-retaining layer that will make irrigation a hellish and long process. Also, wet DE is useless DE as far as killing pests. Only use foodgrade DE. DE for filters (pools and such) is going to have potentially nasty shit in it. Effectiveness of neem mostly depends on local regulations regarding pesticides. if your label does not include "azardirechtin" (or close to that spelling, lol) it is not going to kill any pests. the smell may still repell them and foliar sprays will still have some mold preventative properties - that of horticultural oil and the like. My state does allow the sale of neem oil with azardirechtin. it is "clarified neem" oil only. or the concentrate is techinically 70% clarified neem oil. using clarified neem can still provide benefits. It won't control a problem, though.