I work at a plant nursery and we always tell people to mix 1 part hydrogen peroxide 10 parts water, then water the plant as normal with the solution. It kills the fungus and the gnats will starve to death.

Maybe this will help someone but if you wanna kill everything (and I mean everything) put a clear plastic sheet down over the offending area for a summer. It acts like a greenhouse, heating up and sterilizing the ground, killing the plants (as well as the ground bioculture, but it will recover and killing invasives can be worth it).