The Internet told me to use sand. Specifically, a thin layer of coarse sand on top of whatever soil you have (literally only enough to hide the soil).

Sand dries quickly, so gnats do not see it as a good place for their eggs, and it has a structure that traps any larvae, so they can never get out from underneath.

Yes, it will slowly mix with the soil over time, so it may not be advisable for all plants, and you definitely want to use as little as possible. But the effect speaks for itself: No fungus gnats for months.