This just happened to my club's Twin II Acro as well! Lindner has the part fortunately.

Good thing they do have it as it consists of a solid wire with swaged threaded end fittings riding in a nylon tube unlike the simple cable used by most gliders which would be easy to make! Typical Grob engineering - if someone else has come up with a simple, efficient, inexpensive, reliable way of doing something and it has become almost universal you can rely on them to devise a unique oddball solution to the same problem. I've never got over seeing how the main gear mechanism on our Twin Astir works!