https://www.amazon.com/Stingmon-Pack-Sticky-Fruit-Fungus/dp/...
It's essentially a better looking version of fly paper. They're easy to place alongside indoor plants. Fruit flies and gnats are attracted to decaying organic matter and the moist soil of indoor plants is an often overlooked oasis for them. Gnats are more likely to hang out in indoor potted soils (especially if overwatered) because they feed on fungus, but fruit flies can take refuge there too.
A couple years back I had a months long battle with fruit flies. Kitchen was always clean, garage sealed up tight, no indoor compost, I used apple vinegar traps of all styles but nothing solved the problem until I placed those sticky paper inserts at the base of all my indoor plants. Within a week the fruit flies were gone.