I use insecticidal soap solution or a mild mixture of dish soap and water for both spraying and watering the plants. The gnats have laid eggs in the soil where they hatch into little worms which eventually become gnats. You'll need to be persistent and keep up the soap treatment until their cycle of reproduction has been broken.

Mix this up and spray it on the orchid. Then put the entire plant in a plastic bag--tie tightly--for several days. Remove the bag and inspect the plant--if the gnats are gone, dump the planting medium, wash the pot and spray it with some bleach. Let it dry, add fresh bark and repot. If you still see the gnats, repeat the process. I have never had them make it through 2 treatments.
Mix up: 1 cup rubbing alcohol, 1/2 teaspoon liquid dish soap, 1 teaspoon Tabasco, and 1 quart water.
This works great on mealy bugs also.