Yes, there in fact was a parameter that was denying the SensorGps messages to be picked up by the EKF2, and that was the vel_ned_valid = True. Doing that fixed my issue.
Interesting though, I had disabled the 3D velocity fusion from the parameter EKF2_GPS_CTRL, but for some reason EKF2 was waiting for the velocity to be valid, despite not fusing it into the algorithm. Could it be a bug?