Hi...
As many IITs are calling for PhD after B.Tech and students who appeared for GATE and could not make it to M.Tech in IITs usually called for PhD exam and interview.
There is no such fixed CGPA . Students are shortlisted for PhD interview (For departments who do not take written tests for shortlisting) on the following basis:

Either GATE score or CGPA in M. Tech & B. Tech for candidates who have Masters degree and are applying for regular PhD. Though passing GATE is compulsory if candidate is shortlisted by CGPA (In this case GATE score needn't be current. GATE on which candidate has taken admission in MTech or any other GATE score is also valid)

Cureent GATE score for candidates applying for direct PhD. Generally those who have only BTech degree apply for direct PhD but candidates who also have Masters degree can apply.

Now coming to CGPA.

As I said earlier there is no fixed CGPA for a particular department and it varies each year depending on the following factors:

Number of PhD vacancies.

Number of candidates who have applied.

GATE score & CGPA of candidates who have applied.

Number of candidates department is planning to shortlist for interview. (Ratio will definitely be greater than 1:40)

It depends on the department also (You have not specified department). In few streams it is easier to score as compared to others.

For the branch like Mechanical minimum CGPA is around 8.75 (not exactly) and it is higher for CS, EC and civil.

Hello,
One needs a CGPA above 8 to get into phD programs in B.Tech, IIT, etc courses. However, the minimum CGPA requirement may vary from institutions to institutions and subject of choice. Having cleared exams like NET, GATE, etc. will ease your chances of admission into phD programs manifolds.