Hello student. Ofcourse you can. Mtech in a different stream, btech in a different stream is not a problem at all. It all depends on your interest

But for that you need to clear GATE with really a good score since computer science is a demanding branch

This is a shift from mechanical to computer science which are entirely different.

You can score a good mark if you prepare well. Learn the courses required and know the syllabus. Plan accordingly and you will be able to crack it