I also have a gnat problem again this year. The only thing that worked for me was to take a 9 oz. or larger glass, make a funnel out of paper (computer or notebook) with a tiny opening at bottom. Make the top (opening for gnat entrance) large enough to fit in the glass and use tape to hold it in place. I found they will find a way out if the bottom funnel hole is too large and or if the top part isn't taped securely around the glass.
Pour enough apple cider vinegar in glass (before adding funnel), to get it as close as possible to bottom of funnel. The smell of vinegar attracts them to the funnel and in they go and can't get out. You'll see them walking around inside the glass trying every way possible to get out. They'll get tired and drown in the apple cider vinegar. Place these where you see them the most, in the kitchen, bathroom, laundry room, etc. They like moist areas. Remember to redo every couple days. I tried plastic cups, but for some reason, they go to the glass better. Good luck!
By adbdnc from NH
I wonder if this, or similar, would also work outside for no-see-ums and gnats in the grass. We have a terrible problem. Another thing you could try is take a 2-liter bottle or plastic gallon with apple cider vinegar in it as attractant. They will climb in, but cannot get out the narrow opening, I'd bet. You posted a great tip, which I will remember and try.
You can just add a few drops of dish soap to the apple cider vinegar and you do not need a funnel. I use glass baby food jars.
We have done the vinegar and soap with limited success. We tried one of the old fly tapes you pull out of the cylinder and thumbtack to the ceiling. It works like a dream, it gets them all continuously until you find the nest.