Fungus gnats do not usually cause fungus on plant leaves. The leaf hopper is the likely suspect, as they are sucking pests that puncture the leaf surface and suck plant juice. However, seems that you would be seeing a whole lot more of them if this is truly leafhopper damage.

A more worrying aspect of leaf hoppers and other "sucking" pests is that they carry and transmit plant viruses. Plant viruses are incurable and although they may not cause the death of the plant, the infected plant will then act as a vector for the virus to spread to all your other plants. I'm not saying that's what you have, but giving you info on pests and diseases they carry.

Fungus gnats do adversely affect plants (mostly very young plants) but not by marking leaves and not to any great extent that I can detect, unless the size of the population gets out of control. I do accept a certain number of gnats as a price to pay for growing in the summer, and I use passive controls which work for population control but not eradication.

Maybe I missed it in your post but are you growing inside or out?