Is it possible to calculate GPM of flow if I know I have 13psi in a 1 1/2 Schedule 40 PVC pipe?

you're going to have to calculate the lost work due to friction and changes in pressure and height on each of the seperate pipes, add them up, and then calculate for the main pipe. Then find out how much work is required, you can find that and then looking at charts you can determine an impellar speed at the flow rate you want