For IPhD in IISER you should have 6.0 CGPA or 55% marks in your bachelors. Now mainly the students are shortlisted on the basis of JEST and JAM ranks (as per the official website of IISER PUNE). Its specified that the rank of the student must be better than 500 in JAM or JEST.

*You should visit the official websites for more info.

I hope my answer helps. All the very best for your future endeavors!