The gnats or Biting Midges we have here in SE Louisiana hatch in a salt or brackish water environment. They are usually much worse at dusk and dawn. They are somewhat seasonal here, being worse in the spring and in the fall. Gnats are a miserable pest. Unlike a mosquito, which uses a needle-like protrusion to suck blood. The gnat tears and rips at flesh to get you bleeding!
A 30% Deet application will repel them to an extent. But here, they will literally swarm in clouds at times. We hunt hogs during fall and in our mild winters. I've already seen pigs show up with gnats so thick their eyes looked like ancient Egyptian makeup and their butt-hole looked like it had a black furry tennis ball stuck to it!! I've already been in my stand and, the gnats so thick, I couldn't hear when the hogs were approaching. We wore thick clothing and the head nets with the finest weave we could find.
You might could make sure, just like you would for mosquitos, you don't have any standing water around your place for them to hatch in. Here, everywhere is standing water. For hurricane Katrina we had 14 feet of standing water in our house.
I remember my Dad and grandfather would hang sacks from a wire, drenched with spent motor oil, to drag across the backs of the cattle, to help the them deal with the flies. When the flies were at their worse, the cattle would literally constantly circle through the sacks. I used to think it must've been exhausting to have to keep circling like that to get relief from the incessant, miserable pests.
I'm sure Deet is too expensive to soak sacks or rags with. But you might try a using a few things soaked onto rags for them to pass through/under to get some relief? Just having something to brush past their body, wiping the little devils off may give them SOME RELIEF?
I wish I had better solutions for you but.....
Good luck!