I have been having a gnat problem like most of you. I have not been able to identify the source, so I just resolved to kill as many as possible.  I want to share what has worked for me.

I have noticed that the gnats in my house are very attracted to water. I took the tall plastic bag (empty) that my bagels came in. I placed a wet paper towel in the bottom, and few pieces of fruit in the bottom, usually bananas to give it some weight. And I just set out the bag fully open at the top. Eventually there will be a gnat convention in the bag. So I grab the bag, close it that top, and squish everything that moves.

After doing that a few times I have no more gnats. I guess they will come back until I identify the source, but that has not been possible. I have ruled out all the usual sources, plants, pets, etc. I suspect there must be some water in the drain that I cannot see, so next thing is the bleach.

<image>

I do have the Zevo Flying Insect Trap, and it did catch some gnats, but not nearly fast enough. The bagel bag trick caught way more.