I don't have the time to Google it and find out for sure, but I'm pretty sure that there are absolutely no benefits to having gnats (i.e. fruitflies) on this planet. If I could have chosen the creatures that God left off his to-do list, they would make the top 3 for me. Okay that was mean. But gnats seriously annoy me. In the summertime, here in the Southern U.S., and I'm sure, many other places, they somehow appear, explode into hundreds more each day, and think that you've purchased fresh produce purely to welcome them into your home.
I, personally, got fed up with them zooming around past my ears and face, playing tag in front of my computer screen, and landing on my food. So the past few summers I have experimented with a few different concoctions found by Googling "kill the bastards"- kidding- I don't need the government knocking on my door. I just searched "getting rid of gnats", and found a bunch of variations of a basic mixture. I guess the mixture you choose could be determined by whether you have bougie gnats, country gnats, ghetto gnats, or whatever- no one mixture seemed to work for everybody who discussed their infestation online. After a bunch of trial and error, I worked out a mixture that's finally doing the trick for me. The mixtures were all simple, without hard and fast measured amounts, and the most common denominator in all of them was apple cider vinegar. Some people recommended just putting some wine out, in which the gnats start a party, get baked, and drown, but why pay good money for wine and serve it to gnats? The most typical mixture I found included the vinegar, mixed with a little dishwashing liquid (which, according to some of the posters, made the situation too slippery for the gnats to escape). The mixture is placed in a little bowl or cup out in the open. I've tried it, and it worked okay, but I basically had tiny swimming pools with one or two dead gnats in the bottom, and 4 or 5 still flying around, oblivious to the end I was trying to bring to them.
So recently, my mom gave me the idea to try adding a tiny piece of fruit, since that was their thing. That turned on a light bulb, and being tired of the messy, smelly little bowls, I decided to rig something else. I grabbed a couple of empty brown apothecary bottles from my studio. Carefully poured apple cider vinegar in, filling about half way. Then I added the dishwashing liquid, till the contents reached about three quarters full (don't fill all the way and make it easier for them to escape!). Then, I added a tiny chunk of watermelon to one, and banana to the other. Lastly, I smeared a little banana around the insides of the bottle mouths. My reasoning:
a) they were attracted to the smell of apple cider vinegar, but maybe
the smell of the fruit they can't seem to resist will seal the deal b) dark amber bottles might be confusing to them and meant I didn't have to see dead bugs c) these little bottles were neater, better looking and not smelly like little bowls of vinegar. 
So I put my little bottles out, gave it a day or two, in which the gnat population seemed to dwindle, and low and behold, with a hard look at the bottles held up to the light, there was my proof. They worked! So I thought I'd share it here, where it might help someone else. Hey, I may not be able to save the world, but I got some ideas, lol.
SN:
the raffia seen in the pictures, tied around the bottle was just for
aesthetics, to make them look less like bottles of crack rock or
something. Say no to drugs. Except Claritin, if you have allergies like me.
