So glad you got a diagnosis you can treat. The one thing to remember with ulcers, is that you need to treat them for a long period of time. The horse may act much better, but it's important to treat them long enough to really clear them up!
As Margo said, I would also vote for the 300 mg tablet, and crush it. I mix mine in Maalox, and just syringe it in the 3x per day. It's best if you can keep the dosage times 8 hours apart -- as it keeps a steady dose in the system and does its work properly.
I'm so happy that Eagle will be feeling better soon.
And Gastrogard is expensive, so I also use the cheaper, human version and make adjustments for the dosage. But I still crush it, mix it in the Mint flavored Maalox (which my horses seem to like) and just dose it in the back of their mouths.