GPM depends on just about every detail of your plumbing. The pump pumps with a certain force, which is resisted by the friction of water flowing through the various pipe fittings. The total flow depends on the total amount of resistance encountered.
You can get a rough estimate working from the pump model number, the pipe diameters, approximate length of pipe runs, number of pipe runs in each direction, and the clean filter pressure.
In practice, knowing GPM isn't usually all that useful. The correct pump run time depends on how much it takes to keep the water clean, which is only roughly approximated by calculating turnovers.