The G100 does cater for spindle control. It produces a 0-5V analog voltage from the speed setting in Mach3. The voltage that is output is a proportion to the maximum spindle speed. As an example.
Max spindle speed is 2000rpm.
Mach3 commanded speed 500rpm
G100 voltage output = 500rpm/2000rpm *5V = 1.25V
The G100 generated analog voltage outputs are NOT isolated. So you need to be careful as to how you use it with a VFD or motor controller. Some of these have the 0V reference for the 0-10V control input at around 10V below the mains voltage.
If this is the case, it will cause a lot of excitement and damage if you connect the G100 to it.
I produced a DigiSpeed, the DC-03 that was designed to interface the G100 to VFDs and DC motor controllers.
The DC-03 will take a 0-5Vdc analog voltage and produce an ISOLATED 0-10Vdc control voltage suitable for VFDs and DC motor controllers.
It also has an optional DC/DC converter that is useful as some DC motor controllers can only supply 1-2 mA for the control voltage supply. VFDs can usually supply 10-20mA for ther control voltage supply.
I hope this helps,