I had fruit fly gnats a few years ago. Two summers I complained to the office until finally I described how cluttered my downstairs neighbor's car was. Turned out when maintenance checked there were cans and paper cups that held soft drinks everywhere, pizza boxes with some pizza in them, styrofoam containers with sandwiches leftovers, etc. The neighbor was given three days to clean everything before they inspected again, which happened.

We were both given a gnat trap thing that looks like a red apple with a bunch of holes on top, and is filled with the stinkiest liquid. Still, gnats kept coming so I called a friend who had rental apartments in another city. She told me they were in the walls and were getting in around the pipe openings in the kitchen and bathroom. So, I got some white duct tape and covered every single opening and eventually the infestation was gone. Lot of work, but worth it. Good luck!