Are you aiming for PSU? If you think scoring good rank in GATE only can get you in PSU, then you are wrong. Qualifying GATE will just open the door to enter into PSU, but to get the job you have to face group discussion and personal interview.
The GATE is one filtering to get the cream out all candidates for PSUs. Whenever you are preparing for an interview make sure that your resume/CV should be updated. Highlight your achievements and strength in it. The second thing is your grooming and appearance. Wear light or plain colour dress, keep smiling occasionally while interview is going on.
The group discussion topic is mostly from the current affairs for that you should read the newspaper daily. Even on the day of the interview try to go through the headlines at least. You should not get into argument while discussing, present your points firmly and with confidence. Though group discussions are taken to check your leadership quality you should not speak alone. . In group discussion make sure that you will give chance to others to place their point.
Personal Interview panel generally consists a mix of technical and non-technical members, mostly 2-3 person each. When you appear for interview give respect to the interviewer. Do not be aggressive and you can give diplomatic answers if you did not agree with interviewer's point. If you don't know the answer better say no, rather than giving the wrong answer.
If you are able to qualify for PSU, means you have prepared your GATE syllabus properly. Choose 2-4 your favourite subject and concentrate on it. The interviewer may ask you which is your area of interest. Other than your technical subject you should have an idea about recent treads. Like in CSIT they may ask you about Big Data, AI, Hadoop and other technology. They may also ask you about your projects and seminars, take your project report hard copy with you and go through it once.
You should have an idea about a company, what work it is doing. Before appearing for an interview go through company's website and other details. Know the HR policy, agreement and bond.
They might ask you questions like why you are preferring PSU and not a private company? Why should we select you out of so many candidates. How our sector is better than other.