Gastrin is a hormone produced by cells in the stomach that stimulate the production of gastric acid. There are a lot of different reasons for an elevated gastrin level: just a few are ulcers, pernicious anemia, kidney problems, gastritis, certain medications, and Zollinger-Ellison syndrome (this is a rare produces neuroendocrine tumors). Did they tell you how high your gastrin level was? Do you have a copy of the labs?
The OctreoScan is used to image neuroendocrine tumors. If you have ZES, there's treatment for it. So don't get too nervous yet. I know it sounds scary but you have a while to go before you get any results and you don't want to make yourself nuts!
The scan consists of getting an injection of tracer one day, and then coming back the next day for pictures. Sometimes people have to go back the third day as well for additional pictures.