You’re not wrong Mike - the page does list that as a supported - however while it is a supported message, the “gripper” in the autopilot isn’t actually linked to anything! This is present only because the functionality was present in ArduCopter.
Gripper control is just done via servo signals at the moment, but a future update to the ArduSub firmware may change that.
PyMavlink or even LUA scripts may be a good option to control your vehicle - a secondary Pi also seems unnecessary and a bit wasteful of battery power, given that BlueOS was developed from the ground up to support control software running alongside it in parallel (rather than on a separate single board computer…)