hi,

see gate score is considered for phd admission in IITs, however if you have already completed your masters you are eligible to apply for phd in economics based on gate score. some of the institute has own entrance test which you need to qualify.

some of the best institute which offer phd in economics:

Indian Institute of Technology , Indore

Indian Institute of Technology ,kanpur

Indian Statistical Institute, Kolkata

Indian Institute of Management , Bangalore

hope it would help.