To measure your available GPM, follow the instructions here:
Irrigation Tutorials - Measuring City Water Flow Capacity
Better yet, read through the entire
Tutorial before you get started. It's not the easiest of reads. There is lots to read, and it was written by an Engineer (and we engineering type are not always known for our communication skills). So some of the stuff will be a little confusing (took me quite a while before I could understand what was ment as he tries to explain the difference between a double check valve and a double check backflow preventer).