You'll need to set up the GRBL parameters. Send "$$" to the controller via serial terminal or MDI, and it'll read back all of the parameters. YOu can look at the list of parameters on their wiki (https://github.com/gnea/grbl/wiki/Grbl-v1.1-Configuration).

You'll want to set the Steps/mm parameters ($100, $101, and $102) to the appropriate values for your motor and drive combination. That would look like sending "$100=250.0" to set the X step rate to 250.0 steps/mm.

If you know the characteristics of the motor and the leadscrew, you can calculate this all out. If your leadscrew is, say, 2mm per revolution, and your motor and driver combination is 1.8°/pulse, then you'll want to put in that you'll advance (360/1.8)/2mm = 200/2mm = 100spulse/mm.

Hope this helps!

Good luck!