Yes, Gastrogard and Ulcergard are the same, just have different instructions so one needs a prescription (for treatment) and the other does not because it is for "prevention". You would give the same dose of either one to treat a mini, which is by weight, and often about 4 "notches" on the syringe per day. But whatever your vet told you. And yes, since you normally need to treat for at least a month, order several tubes.
If you know or suspect ulcers there are other things you can do to help. No sweet feed, which aggravates ulcers. Some grains might be OK, but I like alfalfa cubes (soaked) as an alternative to grain because they are high in protein, but mostly are high in calcium which has a beneficial effect on the stomach (think Tums). If you can't offer more frequent hay feedings, consider a slow feeder hay net such as the Busy Snacker, which is more like the way nature designed horses to eat - all the time. For a bad case of ulcers, ask your vet about Sucralfate, available by prescripion but much cheaper than Ulcergard. You can use both, but they can't be given at the same time because Omeprazole (Gastrogard or Ulcergard) inhibits the production of stomach acid while the Sucralfate needs acid to work. The Sucralfate (tablets that dissolve easily) should also be given before feeding, and works by healing the mucosal lining that has been damaged by the ulcers.