Fungus gnats are annoying and best got rid of but the damage on your leaves isn't caused by them. Getting rid of them is good but you still have a problem there.

Fill out a Troubleshooting form from the top of the forum and include anything youve done in the past few weeks eg) adding epsom salts. It looks like a plant that is locked up either by ph issues or a toxic salt buildup.

More pics and more info... include a pic of whole plant. Does the problem start at the bottom of the plant? Is it effecting the entire plant? Things like that are vital for a diagnosis