If the French toast contains only plain ingredients,(egg, sugar, milk, peanut butter) a small quantity will not harm your dog.
However, some dogs will be allergic to wheat and grains. Also, other ingredients like maple syrup, pepper, cinnamon, or any spice added to the French toast will affect your dog’s bowel and digestive system.
If your dog has digestive disorder history, refrain from feeding French toast to your dog.
Moreover, toast doesn’t have that much amount of required nutritive value your dog needs.