First of all, what exactly is gluten? Gluten is a protein found in grains such as wheat, barley and rye.
Humans who suffer from an intestinal illness called celiac disease cannot tolerate gluten which causes inflammation in the small intestines of these sufferers.
Irish Setters are one dog breed prone to celiac disease.
Some dogs are grain sensitive. Dogs were originally carnivores, but due to the economics of making dog food, grains were introduced to keep the costs down and many low cost dog food is primarily made from grains.
Grain allergies are common in dogs. If your dog is scratching a lot, have him evaluated first by your vet before you diagnose him yourself! There are many reasons why a dog could be scratching.
If your dog does require a gluten free diet and has grain allergies, instead of trying to figure out what is and is not healthy for him, you can use this dog cake mix from K9Cakery.com: