I really do not like controversy but drying out soil is just plain stupid (IMHO) and not necessary to control gnats or flies...Should you insist on drying the top of your soil then bottom water by way of a planter saucer and allow water to wick. This is the least effective way to treat the problem...Neem Oil next in line for least effective...Followed by sand applied to top of soil...Next comes diatomaceous earth which works fine over the short haul until it gathers moisture or blown off by way of circulating fan...An insecticide soap is not only effective but beneficial...Cheapest as well as effective and beneficial is good ole baby shampoo...Do not allow soil to dry out...FWIWif they do get in the soil, you can let the soil dry out im sure it kills the eggs of some sort. GOOD luck man!
Still a good thread...May your only problem be one fruit fly (g)...FWIWthere not gnats i saw just a harmless fruit fly in my house that got in to the room with the tent in ,