We used this page for the workshop.
The English Farm has plenty of other GCAS resources.
You can see samples of the test on Youtube
B1: https://www.youtube.com/watch?v=G659sFvdv9A
B2: https://www.youtube.com/watch?v=U7qnCaZhWL8
C1: https://www.youtube.com/watch?v=dBLDBFKIwgM
Videos of the workshops are attached to this page.