Hello @Colinp, welcome the group Colin!
I have a spare Arduino, that I flashed with GRBL for this purpose. I mainly use it to answer questions about gSender from my design computer which is not connected to my CNC.
So if you have an Arduino you can flash it with GRBL and connect to it with gSender. You can then jog around, load tool paths and run them etc. without being connected to a CNC. Some things will not work like homing because there is nothing connected to the Arduino to provide signals but you can definitely play around and practice with gSender without a CNC.