There is no pre-req exam. Everything you would need to know to pass the GCIA is covered in the class. This is one of the harder exams.
I've taken 8 GIAC tests and for some reason I still think 503 was the hardest. Maybe I slacked on the prep for this but it wasn't easy. But like the others have said, the course was great, it's one of the few courses that I've taken that I would like to take again.