diatomaceous earth

Also in my arsenal is diatomaceous earth - sprinkled on the soil (bottom water only if doing this). It will kill fungus gnats and anything else that lays eggs in the soil.

I use fertilizer, so my garden isn't organic, but I never use pesticides other than an occasional application of diatomaceous earth when the caterpillars are getting really enthusiastic...

Diatomaceous earth sprinkled on the soil should help.

Get food grade diatomaceous earth. It’s even pet safe! ... I did this to all my plants and haven’t seen a single pest since.

You can simply deposit this on top of your soil and it will slowly make the gnat population go down.

Diatomaceous earth is my best friend when it comes to insect pests.

I also sprinkle a little diatomaceous earth on top to kill anything that does make it near my plants. More comfort than guarantee, but still worthwhile.

I've had success using diatomaceous earth. I let the top layer of soil dry then put a layer of the earth on top fully covering the soil, and also fill the drainage holes/saucer with it.