A little googling gets the answer.... the MM2001 interface that GMFC interfaces to uses standard step/dir signals for the motors, but has a heat control interface to manually and via PC set and turn on/off the heat and validate the heat is on. This needs to be replicated alongside your stepper driver. It doesnt look too hard to do, and I think there are some examples on the GMFC site. When I get a moment later this weekend I'll have a look through the circuit diagrams and see which one is easiest to replicate.