hey,

this problem seems to happen with every one who are in the role in field engineering position. see to switch from one career to other is really difficult and requires a lot of preparation. one thing I can say you can switch to some design engineer job if you have certain working knowledge of electrical or mechanical design apps. but this also difficult and requires hard working. try to give some other job interview in private sector also where field working is less.if you get low salary than ongc, join in that company not thinking about salary. because health is wealth. I can not tell any thing about preparing for gate examination cause it lots a lot of time and every day you need to study at least 6-8 hours. try to apply in state level government jobs also.