I use ground neem seed meal in my soil for soil based inseckts keeps them all down good. Not sure about fix either damascus earth or neem meal sprinkled on top. Trim stems up and use trap or glue paste
You can use Diatomite...also known as Diatomesceous Earth...on the top soil then water it in.
Living Soils Diatomite worked a treat when I had fungus gnat larvae. After using it once I haven't seen them since.
Yellow sticky tapes are good for the actual gnats.
Good luck