GSP? Don't recognise that, sorry.
I don't know a good source of worksheets but I think I wouldn't expect to at that level - seems to me that maths at that level has a higher explanation-to-routine-exercise ratio than earlier, and therefore you're more in the zone where textbooks are appropriate. I can mildly recommend the Collins Cambridge IGCSE book, and then all the Bostock and Chandler ones for A level stuff.
Ah, come to think of it, you might like to look at Manga High. It totally annoyed me by taking away the nice interface for topic choosing from people with the kind of registration I had, but I think you could register as a "school" and get access to it. The "Prodigi" quizzes on particular topics are quite nice provided you have a way to pick topics that are really appropriate for your child, and there are a lot of them.
