No, grbl is the firmware that implements a subset of the full set of LinuxCNC gcode on lighter weight hardware. While it may add some extra reporting, configuration variables, and a few overhead commands calling it a protocol is incorrect.
Lighter weight hardware is a bit of a misnomer, I believe the grbl ESP32 on my Zenbot is getting comparable to my LinuxCNC hardware of 15 years ago. But that’s just quibbling