There are full sets of GCSE specimen papers available for both Gateway Science and Twenty First Century Science separate sciences and Combined Science available from Teach Cambridge.
They can be located under 'Assessment, Exams, Practice materials'.
The most recent past papers, mark schemes and examiner reports are available on Teach Cambridge.
The OCR ExamBuilder is an online service that allows teachers to build their own assessments using a bank of reformed and legacy Gateway Science Suite and Twenty First Century Science Suite questions from the June 2014 series up to the most recent but one reformed assessment series. OCR ExamBuilder holds thousands of individual questions with mark schemes. You can register to create your own OCR ExamBuilder account here.