I have friends that do GaP, apparently it's rather easy. I'd go for Classics, since it's more different to all your subjects, and gives you a broad base in understanding literature, art and archaeology, politics, social stuffs. Plus it is an extremely well-respected A-level - by both unis and employers
I would go for Politics - extremely useful subject, and (provided you have some interest in politics) is fascinating, especially if you're allowed to debate issues in class. It's a good subject for debating because you can often link things you're studying with current affairs. It also goes well with your other subjects.
Classics! It fits in so well with Philosophy and compliments Sociology really well too. It's very academic and valued particularly by top universities, as well as being endlessly interesting.
I might seem a bit biased, haha, but I know so many people that have dropped g&p but only a couple have dropped Classics in my college.
Do you have any idea of what you want to study after your a2s, maybe?
It depends entirely on which syllabus you do. I do Both Politics and Classics, but I now dont relaly like either of them, because of all my subjects. I cant remember which syllabus I'm on for classics, but its extremely hard. Most of the course is studying sources, and LOTS of them. I'd reccomend politics.