GATE

This is really a complicated question because both GATE score, as well as a percentile, are relative terms. These are measures of performance of a candidate in GATE based on the performance of other candidates appeared in the GATE exam.
GATE percentile represents a candidates performance compared to all the candidates appeared in that same paper in the same year. Whereas, GATE score represents a candidates performance in comparison to all the candidates appeared in GATE exam since 2007.
GATE score and percentile change with the standard of GATE paper. If GATE paper is easy then most of the students get good scores but the all India rank will be high too. If the paper is tough then with less score one can get a very good all India rank.
Now, comes the real question. How much GATE percentile or GATE score is needed for M.Tech admission to IITs? The answer is simple. We need to find an absolute attribute to fix it and rephrase the question. A better question is what should be the AIR (All India Rank) in GATE to get into IITs?
To answer this, let's calculate the number of seats in IITs. According to my analysis, there are almost 50~60 seats in each department on average in IITs. And considering only old IITs take admissions for M.Tech, there will be 7X50 (350) seats for M.Tech in IITs for almost every department.
Now, let's assume, few people go to jobs, study abroad and Govt. sector job we can increase it up to 500 AIR. Apart from this, few departments in IITs conduct their own test and interview for M.Tech admissions may allow students till 700 AIR to be a part of the race to join IITs, this risky side and depends on chance.
Now, we have an idea of the rank we need to get into IITs. As we are preparing for GATE we need to know how much marks we should manage to get into IITs. Our average score in the test should be almost close to top marks and we can keep the buffer of 5 to 7 marks. It means we need to score something around but not less than (top marks 7) marks.
Take practice tests and solve previous year questions to get an idea where do you stand. If you are getting less score put a little more extra effort. Just don't give it. Believe me, IITs are worth giving a hard try, you will enjoy the success later.
Do not forget to upvote as it motivates me.
If you have a question, never hesitate to drop a comment, either I or someone else would reply and help you.