You can apply a deet-free insect repellent to your dogs coat. Makie sure to avoid the face and eyes so that no irritation will occur. The repellent should rid your dog of gnats and other insects that carry infectious diseases.
You can try using Avon skin so soft on him the problem with the stuff you buy to keep the bugs away is you can't put it on their faces but the skin so soft you can put on a wash cloth and wipe it on just don't get to close to the eyes with it.
Basil, extract the juice, and rub him down with it, you take a sprig put it behind your ears and the gnats will stay away if you can figure out a way to put behind your dog's ear they will stay away from him too.
Try using a gnat repellent on the dog. Spray it on the dog's coat. Make sure your dog has been washed and cleaned before you spray on the repellent. Try not to get it in the dogs eyes or mouth. You should see less gnats and flies going after your dog soon after spraying.