Hello Pankaj

You can take the GATE Exam after completion of your B.Sc and below are the Qualifying Degrees that  should be completed for taking GATE Exam

1.B.E./ B.Tech./ B.Pharm.

2.B. Arch

3.B Sc. (Research)/ B.S.- Bachelor’s degree in Science (Post-Diploma/ 4 years after 10+2)

4.M. Sc./ M.A./MCA or equivalent

5.Int. M.E./ M.Tech.

6.Int. M.E./ M.Tech. or Dual Degree(after Diploma or 10+2)

7.Int. M Sc / Int. B.S.-M.S.

8.Professional Society Examinations (equivalent to B.E./ B.Tech./ B.Arch.)

For more details refer the below link

http://www.gate.iitg.ac.in/?page_id=49

Regards