I assure you that if you can ride 50 miles (aside from bonking, which at 50 miles is probably not related to your previous evenings meal) AND you're not afraid of hills, you can most certainly make it up GMR.
GMR may be 5% overall but my Garmin seems to think it's mostly 7% with very little over 8 and some as low as 5.
You know, another fun group ride to do would be OC to SD one day this fall.
Catch the train back up when we're done, a la the Amtrak century but minus the crowds.