Thank you for your question, Louise.  Fruit flies and gnats are different species of insects that infest houseplants (and outdoor plants, as well).  Without a photo, an identification is impossible.  I am afraid the best I can provide for you is an Extension article with information on the most common houseplant pests, with control recommendations:  Common Houseplant Insects & Related Pests | Home & Garden Information Center (clemson.edu)

I hope this is helpful.  Good luck!

Louise:  I cannot add much to the information provided in the Clemson article on fungus gnats:  

"Adult fungus gnats are delicate in appearance and about 1/8-inch long. Often they can be seen running across or flying near the soil surface under a houseplant. They are weak flyers and are attracted to light.

The adults do not feed on houseplants but can be a nuisance to people. In severe infestations they are often seen in large numbers on nearby windows.

The whitish larvae (immature forms) of fungus gnats have shiny black heads and can grow as large as ¼-inch. The larvae generally feed on decaying organic material or fungi growing in the soil. The larvae of some species will also feed on roots. This feeding is especially damaging to very young plants. With older, established plants, the initial sign of an infestation is that the plant loses its normal healthy appearance. A heavily infested plant may lose leaves as a result of the feeding of larvae on its roots.

Indoors, fungus gnats are most often a problem when potting soil that is rich in organic matter, such as peat moss, is used to grow plants. It is especially a problem when overwatering occurs.

Control: For plants that can tolerate it (i.e. most houseplants, especially during winter), allow soil to dry between watering. Dry conditions will kill the larvae. Do not allow water to stand in the saucer beneath houseplant containers, and invert saucers beneath plants outside, so as to not collect rainwater. Products that contain strains of the biological control agent Bacillus thuringiensis subsp. israelensis can be applied to the soil of houseplants and watered into the soil for control. See Table 1 for examples of brands and products. Follow label directions for safe use."  The Table referred to is found at the end of the article.  I suggest you track down some Bacillus thuringiensis subsp. israelensis H-14 for their control.  Good luck!

You are most welcome.  We cut down on the tongue-twisting by just saying "BT."