The best revision is to do little at a time. Maybe a page or two a day?
Cramming is a bad habbit and avoid learning large contents of information in one go, because you'll most likely forget alot of it.
If you do a page or two a day and learn it properly (and review back on those pages from time to time), by the end of it all the information should all stay and it also avoids the whole stress that comes along with cramming.
ANOTHER TIP; ALWAYS revise for end of topic tests, that way leading upto the actual exam, you already have alot of the exam prep done, so then its just a recap.
(btw take it a little easy, GCSE's aren't too bad.)