G series 12 strings have an issue where the neck will actually pull forward with the string tension and pull apart at the joint and the fingerboard off of the top. If you see a gap anywhere where the fingerboard meets the top or along the neck joint and your action on the higher frets is enormous, it is very difficult to play, this is likely the issue. They tried to build it so robustly that this would never occur by adding two wooden blocks underneath the top to reinforce which now makes the neck almost impossible to repair. I’m told filing the bridge down can buy you some time and make it frettable for a time. Maybe ten years or so