Is there a way to kill gnats from soil in a plant?

First place a garbage bag over the plant before moving it outdoors to be repotted. This keeps the gnats contained, kill any that fly before returning the plant back in its spot Repot into clean soil with the roots being rinsed after removal. Use a b- vitamin plant liquid before removal and in the first watering to reduce stress on the plant. Do the reporting outdoors. Use the garbage bag to dispose of the old soil. Do not fertilize till plant shows recovery from repotting.

Vinegar,baking soda and dawn

Just dawn and water in a spray bottle will kill them and it want hurt the plants.