It looks like they didn't get the end of blank glued to the grip well enough and it probably should have gone a bit deeper. You said you used Gorilla Glue? Did you get it on the blank also?
If it comes off again, I'd use 5 min. epoxy and make sure you get the end of the blank clean and then glue it as well. Not just your glue line in the cork. Just out of curiousity, are you underhand casting this rod? I only ask because it's an interesting break.
I almost forgot, you could turn a new end out of wood. Rotate the rod in a fast turner and cut the cork down square. Get the blank as clean as possible. turn a new knob out of wood. I'd use a stabilized wood but if you don't, finish it really well. It's not hard to turn the wood seperate of the rest of the grip. I always do the wood seperate because of the hardness difference.