This sounds like fruit flies, not gnats. There was an article about them on spark people:
It referenced a food network article which had even more information:
I get these every fall when I start bringing in my tomatoes. Leaving ripening fruit outside actually can mean they have MORE exposure to these flies, you just won't see them in the house.
They only live a week, and I actually will vacuum them up in my kitchen. I know that they can fit through my screens, so I have been seriously thinking about rescreening them with a finer mesh that these can't get through.
| Pounds lost: 8.0