I keep bananas around 24/7 so I get gnats quite often. One time it seemed like I had 100s of them because I had bananas, watermelon, and other fruit that brouht them in. What I do for ordinary house gnats (may work for fungus gnats also) is pour some cheap wine (the darker the better, I use Merlot or Shiraz) into a coffee cup (about 1/2 full or a little less) and then put saran wrap over the top and make sure it's pulled tight and is big enough to stay secure around the cup. Now poke holes in the top near the center with a toothpick, I usually poke about 10 or so. The gnats will go in after the wine and not be able to find the holes to ge back out. I put the cup wherever the gnats are flying around the most. It may tae them a day or two to figure out where the smell is coming from, but they will eventually go to the cup.
