I buy pyrethrin concentrate at my local grow shop. Add 30ml (2 tbsp) per gallon and give them a thorough watering. It will kill your gnats and has no adverse effects. If you're dealing with fungus gnats apply once a week for 4 weeks as you have eggs and larvae in your medium and they have about a 30 day life cycle.

I've found unidentified critters on occasion and one application did away with them.

It takes a fairly high population of fungus gnats to affect your plants. I've kept them in check by spraying the adults daily, but if you have more than 2 weeks left I'd go with my first suggestion.
grey1223 Reviewed by grey1223 on . HELP!!!! GOT GNATS IN MY SOIL WHAT TO DO??? I MUST OF OVER WATERED ANY NATURAL WAYS OR SAFE WAYS TO GET RID OF HELP!!!!! Rating: 5