I noticed that the leaves of 2 of my tomato plants have crinkled leaves and...
Thank you for your questions. First, as to the leaves rolling, it can either be an infestation of insects causing this or a condition called "leaf roll," described as follows:
"Are the edges of your leaves curling up? This could be leaf roll. It's most often the result of heavy pruning or root injury. Plants may lose leaves but will recover. Some varieties have a genetic trait that causes leaf roll under stressful conditions like cold, heat and drought. Plants with this trait will produce normally despite their appearance."
Second, as to the 'black gnats,' I think they likely are black aphids, which can, through sucking the food out of the leaves, cause leaf roll. Here is an Extension article about them. I am unable to see them in your photos, but you can compare them with the photos in the article. They are soft insects that can be knocked off with a strong stream of water (don't break the plant stems), or are food for the predator insects listed in the article, including lady beetles and wasps.
If you can capture some of the insects on a rolled leaf, seal them in a plastic bag and take them to the county Extension office for identification. Good luck!
Thank you Kristena. I appreciate your help.