Hey Ziggy,
You would not need to create Gcode to control the VFD in order to shut the spindle down.
The UPS shutting down the computer is a common thing, many people, including me, use nut for this (which would be easy to install on (Raspberry Pi 3-based-) Onefinity Controller since it’s in raspberrypi.org repository), and for emergency shutdown of the spindle you may trigger the VFD’s ISO 13849-1 Safety Inputs (or similarly programmed Input Terminals available on any VFD). I showed diagrams here the other day.