There is no problem. Typically, Mach commands the spindle to a certain value. If you set the spindle speed to 1000 RPM, then Mach expects the spindle speed to be 1000 RPM. There is no PID loop from Mach controlling the spindle for external motion plugins, so there is no need for RPM feedback.
Feedback from the galil is in counts because, after all, it reads an encoder. You could possibly write a brain to sample the counts of the spindle encoder and do the required math to calculate RPM based on your encoder counts per revolution. But that is a subject waaaaaay beyond the scope of this Galil forum.
Why do you need spindle speed feedback? If you just want to check that the spindle is doing the commanded speed, then put a tachometer on it temporarily.
Steve