Hello Sourav Pal

Mtech without GATE is a 3 year course while after GATE it is a 2 year course.
Almost every top company which goes to IIT came here with very few exception.
as far as placement are concerned you will be allowed to sit in each of these company. In most of the companies like Microsoft, Amazon etc, there is no percentage criteria.

So for placement in core companies you will have to be very good at coding + knowledge and it is not easy. but in an average company there will be 100% placement.

Hope it helps you.