Dushyant there is a provision of opting a different specialization branch for doing M.Tech from IITs on the basis of your GATE score but whether you can get a CS branch specialization branch with an IT GATE score cannot be said surely because it depends on the various branches department whether they want to accept a different branch score or not. There is no fixed provision and it varies from college to college, department to department within the college.

As Mechanical and Computer science are two totally different departments and among these two generally Computer Science department is rank above in most of the colleges so it's a little difficult to switch from Mechanical to Computer Science department but still work hard trying to get a very good score in GATE which will not only increase your chance of getting specialization in computer science department but will also fetch you a good college.

Feel free to ask if you have any more queries. All the best!!