It's "either..or". Unfortunately, there's no failover mechanism to automatically switch to a default server when the GMS is down. This would add more complexity as it seems at first.
I recommend using the default server if there is a slightest possibility that the GMS PC is unavailable. We constantly monitor our mediation servers and they are located in industrial grade data centers. If they are down (which is very rare and mostly due to maintenance for a very short time) we are notified immediately and can take all measures necessary.