Well I can't really advise you whether a GCSE is much help, but working on the assumption it wouldn't be anymore useful than an A-Level I'd say it's unlikely to be that much use in the test. There are two sides to this story, however. While the specific knowledge you gain from an A-Level isn't of great use with the question you might get, you would probably do well to acquaint yourself with the most common 'philosophical problems', because from my experience that is what the more open ended questions seem to revolve around.
A-Level would go some way to preparing you for this, but on the day there should be no difference between someone who has done A-Level and someone who has read and thought about a book as basic as 'The Philosophy Gym'. I actually got a question on something closely related to what I was studying in philosophy of mind at A-Level, but I made a conscious effort to not cite, or even imply, any of this knowledge. I assumed they were looking for natural ability rather than learned facts, so showing huge background knowledge would be a waste of time and ink.