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 the qualification webpages:
GCSE Chemistry A Gateway Science year 10 practice papers - Login for OCR Interchange required
GCSE Chemistry A Gateway Science year 11 practice papers - Login for OCR Interchange required
The most recent past papers, mark schemes and examiner reports are available on OCR Interchange. These will transfer across to the OCR website in the May of the year after they were assessed to sit alongside the past papers, mark schemes and examiner reports from previous years.
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.