My experience with gnats is indoors, but maybe my experience can inform yours:

Gnats and other flies don't live very long. If you have a lot of them, it means that they are breeding somewhere nearby. This is why I am nervous about restaurants with flies in them: Those flies are breeding on the premises somewhere, and since flies breed in unsanitary places, it means that soemwhere on the premises is an unsanitary place.

I worked in an office once where I would be sitting there minding my own business at my desk, and I would get gnats bothering me. In a standard cubicle farm, in an ordinary modern office building.

Check the kitchenette and the garbages... nothing special, other than that sometimes the gnats would be attracted to the food there. But no breeding places.

Check the potted plants. Hm. A few gnats in the soil of one of the potted plants. Okay...

Hey, there's this coworker with like, a dozen potted plants in his cubicle. A few gnats in his plants, but nothing hugely major. Hm. Look at the last potted plant...

A small cloud of gnats comes out of the soil of that potted plant when I poke at it. The owner of Plant Cubicle is out on vacation for a couple of weeks, so I have no recourse but to place his infested plant outside the office, in the garden.

A few days later, I'm still seeing gnats in my office. That means they're still breeding somewhere. Go back to Plant Man's cubicle.

Under his desk I notice... A big old bag of potting soil. I look more closely, the bag is not sealed. The top of the bag is folded over, but it's not 100 percent closed. I reach out for it like a horror movie victim slowly reaching for the doorknob... And of course as soon as I touch it, a cloud of gnats comes out.

Bag of potting soil gets unceremoniously placed outside. Gnats in my office are gone the next day and do not return.