hello akash

you have to fill an application form for ONGC recruitement through GATE and you will be shortlisted on the basis of gate scores . then candidates have to face a personal interview  and further procedures you wll get to know by tourself . there is a good info on careers360

https://engineering.careers360.com/articles/ongc-recruitment-through-gate

checkout this also

https://www.ongcindia.com/wps/wcm/connect/en/home/

(i am not a gate expert kindly look for other answers as well)