Ahhhh,
You see the damage that IS being done is out of your sight. It is occuring down in the soil, but still close to the surface. That is where the larvae of these fungus gnats dwells. Since you are seeing the flying gnats, you can automatically figure that you have the larvae chowing down even now.
You might consider putting a 1/4 inch thick piece of sliced potatoe on top of the soil, just simply lay it on the top of the soil and give it about 12 hrs, then quickly lift it up and see if you have any 1/4 inch long critters desperatly trying to crawl away to hide.
The fine dusting must cover all the soil, and then it will just slice and dice the critters. Water from underneath also for awhile to allow the dust to stay in place for a lttle bit.The water and this Diatomaceous Earth make things sorta clump up, better to sprinkle and then just leave it alone.
Good Growing
smoke in peace
KingKahuuna