Some past tips have been to let the plants dry out (larvae live in the damp soil), putting a layer of sand on top of the soil to discourage the egg laying, sprinkling cinnamon...none of them worked for me when I brought home gnats in a new prayer plant and they invaded every plant in my home.

What did work was buying a package of mosquite dunks, floating a piece of the dunk in a container of water overnight, then using it to water my plants. The BT in the mosquito dunk kills the gnat larvae and stops the cycle...it may take more than one treatment if the infestation is heavy.

The same BT (bacillus thuringiensis subspecies israelensis) is in Gnatrol, & Knock Out Gnats, probably others...I just found the dunks better priced locally (and passed the rest of the package on to a friend with a water feature in their garden)

Thanks. I will try that.