Hi there, with a galil, you would use one of the axis analog outputs for spindle control...
it will give you 0-10v to run your vfd...
on my lathe, with a galil 2130, I use the Y axis to drive the spindle....the spindle operates open loop, not like the axis drives...
hope this helps....Brian B is working on this suff in the Galil plugin as we speak...he is after rigid tapping, and lathe threading...
as of now, I have the galil wdsk software controling the spindle speed, but not mach3 yet...the rest of the axis motion, and i/o is working great!
good luck with the galil...