Are you talking about that little white(?) spot? No idea but if that's the worst it gets I wouldn't bat an eyelash at it.
https://www.420magazine.com/communi...try-cure-problem-i-seem-create-another.97592/
Some info on gnats in that post. I don't believe the gnats are eating holes in your leaves though. I have a few of them from time to time and have not noticed any damage from them...and since I only have one little plant I tend to examine her pretty closely lol. That thread says that the larvae or whatever can damage the roots but the adults don't really hurt the plants.
As for getting rid of them...get rid of the excess moisture in your soil (i.e., let them dry out longer between watering) or anywhere else in your grow area. The only time I ever have them show up is right after I water for a day or three, once the soil dries up they disappear. What's your watering schedule right now? At the size yours are (looking pretty good btw) in those size containers I wouldn't think you'd need to water more than once a week...every five days or more.
I was looking at the pics you just posted and noticed that it appears to be in a basement so I checked the first page of your journal and confirmed my suspicions. You also mention that it's fairly moist/stale down there...I would be willing to bet that that is also a major contributor to the reason you are having issues with gnats. That particular problem is also brought up in the thread I linked to above.
Hope you get everything worked out!