I'm a little "late to the party" but I just wanted to comment that the same exact thing happened to a riding buddy of mine with his GRR as we were riding on a recreational bike trail here in Central Florida. He noticed that the bike felt "funny" and luckily stopped to check it out. The frame had cracked right in the same spot yours did.
The good thing is that we were riding in a group and one of the other riders offered to go back to the parking lot which was by that time a good five plus miles away, get into his SUV, and pick up my friend and his bike to take him back to his vehicle.
Long story short, my friend contacted Easy Racers and they told him to strip the bike of all the components and ship the frame to them so that the crack could be repaired. He did just that and in a few weeks, he was riding his beloved GRR again. By the way, this actually happened more than 15 years ago. How time flies. I'm not even riding any more but hope to change that sometime soon.
Glad to see that your situation has had a resolution.
Cheers