Looks like it also have an encoder or resolver built in. Look for rated speed on the label, if nothing on the label, I would guess 3000RPM. Run it up to this speed using another motor and measure the voltage generated by the motor. But really I think max current is more interesting than max voltage. If you know max current, the drive will use the voltage necessary to bring it up to this current. And if this is a small router/mill your motors seems big enough that it would very seldom be driven hard anyway. Your drive will only send enough voltage and current to your motor to keep the following error low. Assuming your PID is correctly tuned that is.
Another way to go is to hook the motor up with a load and test with max continous load and judge the max current setting by how much it takes to heat the motor so it is not comfortably touchable after 30min of this "abuse".
Other than that I would first of all try to contact Gecko support. From what I've seen (I'm not a customer) they are concerned about their customers also *after* the sale is made.