My universal gnat advice: get a cape sundew, put it in a plastic tray of distilled/RO water on a sunny windowsill, finally enjoy seeing a plant covered in gnats.

That's...pretty much it. They must be kept in non-porous pots and trays (glass or plastic), water TDS must be below 50 ppm and keep water in the tray constantly. They will even bloom with gorgeous little flowers. No fertilizer ever, just bugs.

I can't emphasize enough a sunny spot. This window faces southwest and so it gets 8 hours of direct sun. This helps with growth and also makes the mucus shine which seems to attract the bugs.