Engineering Eligibility

Most of the institutes (All IITs, NITs, IISC & other reputed colleges) use GATE score for admissions in M. Tech programmes. Some institutes (Mostly IITs) have the provision of 30% of marks in BE/B. Tech in final allocations of seats.
In MNIT, GATE score is used for admission but min. 6.5 CGPA is required (due provisions for various categories).
Detailed admission brochure is here-http://mnit.ac.in/cms/uploads/2015/04/Admission_Brochure_odd_Sem_2015-16_revised.pdf
However few institutes have their separate procedure for admission in M. Tech programmes. You can read about that in following link- http://www.inspirenignite.com/m-tech-admissions-2015-without-gate/

Hey,
Working at Google, Facebook or any other such IT firms does not require B.Tech from computer engineering as eligibility. All you need is to get yourself prepared in computer and information technology. Whether you do B.Tech or B.Sc. it doesn't matter much. Google hires people from non technical background who have done graduation and post graduation in mathematics and analytic for their research purposes. But if you do B.Tech. it will only make your path a bit easier, but not much.
As you mentioned that you want to go ahead in field of research, you can opt for B.Tech. but you have to do your post graduation either as MS or M.Tech. in field of your choice in which you want to do your research.