The Geckodrive G540 has a optically isolated VFD output that can be configure to run spindle speed controllers on some popular import mills and lathes.  The VFD utilizes a PWM control via pin 14 of the DB25 connection.  You provide it the 0V reference and plus 10VDC of the speed control circuit, and the VFD out give you a 0 to 10VDC based on the duty cycle percentage of the pin 14 signal.  The X2 mini mill for example does speed control with a 5K potentiometer.  One side of the pot is reverence to 0V of the mini mill speed control 12V supply.  The G540 uses a 16V op amp to drive the vfd, so you can use the 12V supply as the VFD supply input on the G540.  Essentially the only modification needed is to remove the wiper connection of the potentiometer to the control pcb. This is done at the connection to the controller pcb.

Later models have a switch on the potentiometer that must be closed before the speed control setting can be made.  For simple remote control, it isn't necessary to emulate that switch as the switch contacts simply latch the motor relay.  Once latched, the switch has no circuit effect, so you can just leave the switch closed, it HAS to be closed for this mod to work.  Below is the high level schematic of the mini mill wiring with the 3 wires that will be involved in the connection to the G540 VFD.  I would suggest you confirm that your 12V and 0V wiring is as shown below before modifying:

Below is how the G540 is connected to the speed controller.  You only need to remove the wire from the potentiometer to P2, the other wires of the potentiometer can stay. 

The above configuration is the simplest, but it eliminates using the potentiometer on the mini mill manually.  If you want to maintain that capability you can insert a spdt relay between the potentiometer wiper wire, the VFD output and control it via the G540 and an M3 command setup in MACH3:

G540 MACH3 XML File

Here is the MACH3 mill xml file with the G540 settings including the VFD setup which at this writing isn't in the file on the Geckodrive website.  In you browser do a "save as" http://pminmo.com/misc/540BVFD.xml

The particulars of how MACH3 setup interfaces with the PWM circuit below, the Minimum PWM value can be adjusted based on your particular machine: