Did you start the GCSE content in Y9? If not, then there's not much point going over what you've already done. The summer holidays is quite a lot of time btw!
You pretty much recap everything in Year 11 anyways so don't worry. You would be surprised by how much a teacher can cram into that last year. I wouldn't begin proper revision just yet, but feel free to.... Christmas is when most people started in my Year... I didn't start until Easter lol.
Did you start the GCSE content in Y9? If not, then there's not much point going over what you've already done. The summer holidays is quite a lot of time btw!
No it's never too late to go over the important stuff. I recommend you go over the ones you are weak at first. Getting revision cards done now will be helpful for you in the long term