There's a difference between fruit flies and fungus gnats. Fruit flies come in on store-bought fruit and veggies in the summer time, and feast on fruit as well as fermenting tomato seeds! They multiply every day, and have a blast in the compost pile where they fly in your face every time you lift the lid
Fungus gnats come in on cheap and nasty potting soil that has been left outside for months at the big box store to get wet! They dine on tasty young seedling roots
Linda