I have no experience with problem to the plants due to gnats. However a few resources online claims that the larvae chew on young roots.
"Fungus gnats are small flies that infest soil, potting mix, other container media, and other sources of organic decomposition. Their larvae primarily feed on fungi and organic matter in soil, but also chew roots and can be a problem in greenhouses, nurseries, potted plants and interior plantscapes. Adult fungus gnats may emerge from houseplants indoors and become a nuisance."
ipm.ucanr.edu/PMG/PESTNOTES/pn7448.html