If a system is on a vehicle, but not operated unless it is in a fixed position, the more conservative approach would be to treat the environment as GM because the system is still exposed to the levels of vibration experienced even though it’s not operating, That is, connectors can loosen, particles can shift which might short when the equipment is turned on (assuming it’s electrical), etc.

You could also divide the “mission” up into a GM and GF segment, then calculate the weighted system failure rate.