Of course the GCSE system is the exam system for Britain and so is different from any exam system of the USA. Typically depending on what level of GCSE you are taking, you will find that there are two kinds of questions to respond to based on this novel. The first will be a question that refers you to a specific passage and then asks you to write an essay on that passage. The second will be a more general question referring to the novel as a whole. In your revision you need to be ready to answer both of these questions.
For example, a very good question that refers to a specific passage would be something like:
Referring to Chapter 1, discuss the animal imagery that is used to describe Lenny. What impact does this have upon his character?
This question requires you to go back and look at Chapter 1 and obviously pick out and discuss the way Lenny is described (pick out descriptions of him as a "bear" and certain actions etc. that he does that support this presentation of him as an animal). Then you need to talk about the impact this has upon his characterisation as a whole.
The second question would normally refer to a specific theme of the novel or a specific character, asking you to discuss how that character changes or the theme is developed. For example, you might get a question like this:
Discuss the treatment of loneliness in this novel.
This kind of question would involve you thinking through this theme and how it is presented and then talking about what the novel is trying to say about the theme. For this question, you would obviously want to talk about the fear that so many characters have of loneliness and how so many of them are lonely. Another question based on character might be:
Discuss the role of Curley's wife in the novel.
This kind of question would involve an analysis of her role in the novel, considering her actions, her words and her interactions with other characters. Does she represent anything? How is she used by the author?
Above all, the best way to prepare for any exam is to be very familiar with this novel. Good luck!