Unknown sick and dying orchid...SOS!

[silken]

You can use sticky tape to catch the adults but the larvae live in the media. You can get fungus gnat spray but it likely won't help with the larvae. I myself would either soak the media until they are drowned, or use an insecticide spray on the media and hopefully kill them that way. I like Malathion but it stinks. Since this plant should be re-potted (in my opinion) into a smaller pot. I would be tempted to dump that media. Clean all the roots and pour boiling water over some new stuff or the existing media, which should kill the larvae. Then re-pot into an appropriate pot.

[Carpe Diem]

I got rid of fungus gnats by putting out some apple cider vinegar for the adults and killed the larvae by adding some dish soap to the water. I watered twice with dish soap added, just to make sure.