Fungus gnats eat organic matter and maybe fine root hairs; it would take a sizable infestation to kill a mature plant although seedlings and small clones could be affected. You might have gnats or some other flying insect, but you also seem to have some other issue(s). Picture of the whole plant and details about the environment would help us help you better.
Fungus gnats in soil can be treated\prevented with Bacillus thuringiensis or Bti bacterium that produce a toxin which will kill gnat larvae. Bti is now more commonly available in liquid formulations, even from Lowes\Home Depot. Bti kills mosquito larvae as well and that was its original commercial marketable indication; Mosquito Dunks can be found everywhere if you can't get a liquid formula, break up the puck and soak about a quarter of it in a gallon of water for 12 hours or so, then water.