These are pure governmental organisation and often termed as psu.The recruitment is through gate and there own entrance (if possible otherwise it is mostly through GATE)followed by a personal interview.These research organisation hire candidates of all branch according to vacancy.The competition is quiet tough not only because of large number of aspirants but also due to entrance exam syllabus.

In btech minimum marks usually asked is 65% or equivalent CGPA(6.85 this time).

So if you score well in GATE there is fair chance you might get a call from these organisation for personal interview.

Good Luck