I have heard aloe vera juice working really great for ulcers. I never used it myself.
My vet worked with me on the not wanting to spend that much on Gastrogard either, so she gave me this external wound cleaner called Draw (which I cannot find anywhere online..) that worked. She explained it would work because it is high in magnesium, which heals ulcers.
I second the Aloe Vera juice. Also Papaya Juice, I think its called Stomach Aid??? Kinda spendy and you have to keep it in the fridge. I am sure you might find a less expensive way to find the Papaya juice.
Too bad your horse is allergic to Alfalfa...that is supposed to help as well.