I myself did many organized rides from 92-2004. I actually got burned out having to time my "training" and prep for those specific days, as if I had to ride. I rarrely do an organized ride anymore, we just ride.
Not to mention, you haven't done GMR (that I recall). There is a reason we and several others do it, it's just a great cycling experience and adds another dimension to riding. Do it, you won't be bored and you will want to do it again, and again. I tell everyone it's a different attitude up there, all the riders are encouraging. When you make the village, you'll want to do that again and again as well.
I've seen you do Baldy Rd and you are a very good climber, GMR won't be much of a challenge but it is a different experience. You can see in the video the guys passing us at the start are very friendly (not in our group). GMR riders are more of "you can do it" type riders vs the "I can beat you" type riders we meet on the lower lands.
It's a different experience, ask any one of the Clyde/Athena forum riders in the video
21 mile climb 4300ft, 5,000 ft roundtrip. Lots of scenerey and serenity