You’re right, it looks like GSF is advertised as alternative primary source in the documentation.
I didn’t find many info about the EK3_GSF_RST_MAX parameter, Rover-4.1.0-beta7 has been released for beta testing! mentions “EKF3 GSF can be invoked multiple times with source switching (no longer limited by EK3_GSF_RST_MAX)” under bug fixes.
Let’s hope somebody with some “EK3 GSF developer knowledge” stumbles across this thread to enlighten us further!