Either the bacteria BTI, which Mosquito Bits contains.

Or the real fix, SF Nematodes. Nematodes will live in your soil and eat various pests, depending on the kind, and SF are good at eating fungus gnats larvae. Other Nematodes can be used for fleas and stuff too.

https://www.arbico-organics.com/product/nemattack-beneficial-nematodes-sf-steinernema-feltiae

https://www.naturesgoodguys.com/products/beneficial-nematodes-sf?variant=23323522563