I agree with the above, it's piss easy and even if you're not very good it's very possible to get an A* with a minimum knowledge, a bit of preparation, and a brain. I imagine the Oral and Listening sections will be easy for you, as well as probably the Reading (and if not common sense helps here). The Writing I found hard but you can prepare it - the first question will always ask for 5 things, the last question is always a letter, postcard or short essay (so prepare beginnings and endings), and there is a section which only gives you marks for accuracy (so just use really basic Chinese) and one which tests how good your Chinese is (so use one/two impressive structures and some impressive vocab which you can learn the day before). I'm crap at Chinese and scraped an A*, and I wouldn't have been able to do it had my teacher not taught us how to play the system. You've been to Chinese school for 6 years and are probably better - I would say ask your teachers about the GCSE and then do a bit of vocab learning.
P.S. I would say it's easier than Japanese GCSE