It depends on the drive you are using, if the drive has encoder inputs, it can be for commutation or tach feedback.
Often a drive of this sort would have an encoder output that could be used back to the Galil controller.
Find out if the encoder input is optional or not to the drive.
The bottom line is you need the encoder(s) to go back to the card if dual or single encoders are used.
There are some AC drives that have resolver feedback from the motor and output a pseudo encoder output that is available for the CNC controller.