Purina Outlast DOES NOT have Gastrogard (Omeprazole) in it.
I like the idea behind Purina outlast but you should think of it as more of a Tums for horses than any sort of treatment or even a preventative. IMO. I do have a bag of it that I won, and I do think it has it's place with horses that are prone to a more acidic stomach. It neutralizes the acid the same way a tums does for humans.