When I still lived in Montana and was heavily into trail riding and roundups, gnats were often a problem for both horse and rider. We used "Gnat Away" for both horses and ourselves. Here's some info:
Gold Nugget Gnat-Away
Active Ingredients: Herbal extracts and emollients in a vanishing cream base.
How to Use: Apply to affected areas as needed.
Advantages: All natural; nontoxic; nice odor; works well; no oily residue.
Disadvantages: Too expensive to use on entire horses, so reserve it for spot-treating much-bothered and sensitive areas, such as your horse's face and ears; not widely distributed-you may have to search to find it.
Cost: About $9 to $12 for a 16-oz. jar of cream.
Contact: Neogen, 800-621-8829 to order, or for a distributor near you.
I still have part of a tube, and it still works. It's probably 20 years old. Got it from the farm and ranch store where I worked at the time.
Good luck. oldbat