Gnats can be caused by decaying organic matter but they are more commonly related to keeping the soil too moist. Let the pots dry out more in between waterings and treat the soil with neem.
Gnats can usually be controlled by watering with compost tea with 1 oz of orange oil per gallon of water and waiting a longer period of time between waterings. A light dusting on the soil of horticultural cornmeal often helps and heavy infestations can be controlled with a drenching of neem. There are several products on the market but avoid those that also contain pyrethrum and PBO (piperonyl butoxide).
A listener reported this idea. Put apple juice or apple cider in a mason jar, cover the opening with plastic and secure with a rubber band. Stab a small hole in the plastic. The small gnats are attracted th the apple juice, enter the small hole and can't get out.
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