hi,

see since you have already completed your m.sc you are eligible to appear in gate exam. gate score is considered for admission to masters, phd course in India or abroad. iits offers phd admission based on gate score.

you may apply for phd course related with  relevant discipline as per your past course criteria. candidates are eligible to apply in gate exam irrespective of their bachelors or masters specialization course.

you may appear in gate from XL paper. chemistry is compulsory for all candidates, candidates have to select any two papers from botany, microbiology, zoology, food technology and bio chemistry.

follow the below link to know about gate papers details:

https://gate.iitkgp.ac.in/gate_syllabus.html