Hello,
What we call gnats may include true gnats, midges (also called No-Seeums) or true Black Flies. Generally, most species of gnats are light colored and non-biting, midges are grey to dark red, and Black flies are black.

The females of both midges and Black Flies bite because they need a blood meal to produce a substrate for the developing young as they hatch - just like mosquitoes. More than likely, if you are seeing them in the spring and fall they are probably Black Flies. Midges need a moist environment to lay eggs and produce young, where as Black Flies need clean, slowly moving water to develop. 

Black Flies are active for 1 or 2 hours after sunrise and 1 or 2 hours before sunset. So, you may want to avoid being outdoors during these time periods. The good news is that the Black Fly is not known to transmit diseases.

Here are some other things you might want to consider. You may want to wear long-sleeve clothing and cover any exposed skin.  You may want to use DEET or any product that contains DEET such as Cutter-Off since, like mosquitoes they are attracted to CO2 that we expel when we breathe.  You may want to use Avon Skin-So-Soft cream instead of DEET to any exposed body part. And, you may want to try using pyrethrin sprays or foggers as a deterrent. Limited success using insecticidal sprays have been noted.

Good Luck.