One is to treat ulcers diagnosed via a vet and the other is preventative after treatment. Gastrogard in the one that contains the acid inhibitor (omeprazole). See below:
After a horse has been diagnosed, treatment with GASTROGARD® (omeprazole) for 28-days is recommended. GASTROGARD is the only product proven to treat existing stomach ulcers and the only one that is also FDA-approved. Following treatment, an ongoing preventive regimen using ULCERGARD should be initiated.