How do I figure out the gpm from my pump size of 3/4 hp motor with 3450 rpms?

You need a lot more information than that. I can think of a few things you'll need to know: the pipe diameter, the supply head, the head you're pumping against, bends in the pipe, the viscosity of the fluid being pumped, the type of impeller in the pump, etc. I assume you're pumping water so the viscosity and fluid identity are known. You still need more information.

Manufacturers often have charts that will give approximate indications.

Try this:

Pump manufacturers.com calculators (http://www.pump-manufacturers.com/calculators.html)