Candidates who have completed their B.E/B.Tech degrees with a valid score in GATE are eligible for M.Tech admission.

IISc, IITs, and NITs offer regular M.Tech seats to candidates who have been sponsored by their employers. These candidates don’t need to appear for the GATE entrance exam for admission in M. Tech programmes.

Basic Requirements for Getting into IITs for M.Tech.
1. B.Tech percentage must be above 60. Do read my article on this issue 60% aggregate for M.Tech admission.
2. Must have a wonderful GATE Score or Good All India Rank, or Good percentile. Don’t get confused and afraid with the 99 + percentile. The percentage is different from percentile to read more about it refer to How to calculate GATE percentile.
Also, refer to Top Marks in GATE 2010 to get an idea of how many marks you need to go to IITs 800+ score may reasonably take you to IITs.
3. Be prepared to take another test at IIT for admission. Learn the basics of each subject, especially core subjects related to your engineering stream, as there may be a written test for admission to M.Tech if the number of candidates has close scores. Written test decision totally depends on the Department offering M.Tech program in IITs.

Procedure to take M.Tech admission in IITs
Candidates have to go through the complete step-by-step procedure given in the above link.

The GATE percentile is calculated as follows:
P = ( (N- your rank)/ N) x 100
Where P is the percentile and N is the total number of candidates appearing for the exam.
(N- Your rank) indicates the number of candidates who have scored fewer marks than you.
For example, if a total number of candidates are 100 and 90 of them have less marks than you P = (09/100)X100 = 0.9×100 = 90 percentile.

For more details on, GATE Score Calculator click on the link.