There's no single answer to what GCSE grade you need in Additional Science to study A-level Physics. Requirements vary between schools and colleges. However, generally, a
strong pass, often a grade
6 or above (or a B grade under the old system), is expected. Some institutions may require a grade 7 or even a higher grade depending on the competitiveness of the course.
It's crucial to check the specific entry requirements of the school or college you are applying to. They will explicitly state their minimum grade requirements for A-level Physics.