Dear Ghanshyam,

For admission into old IITs (Bombay, Madras, Delhi, Kanpur, Kharagpur, Roorkee etc.), 650+ is a safe score. Chances of getting M.Tech in IITs at 639 GATE score is less, however there something else you can do which can help you get sure shot admission into IITs.

There is something called M.S. TA and RA (TA- Teaching Assistantship, RA-Assistantship), the cutoffs for these are usually lower as compared to M.Tech programs. You will be called up for a test and interview to individual IITs, if you perform there, you can get admission into IITs, possibly into IIT Bombay also. Sometimes they don't even take up and interview and directly give admissions on the basis of test conducted.

Please go through the official websites of all the IITs you want to target and look for M.S RA programs to shortlist the ones on your choice.

Howerver there is a small catch here,

the MS RA program is a 3 year program. You will be engaged in coursework for 1.5 years and for rest 1.5 year you will be working on some open ended research problems.

The placement is senerio is exactly the same, and if you want to get into R&D department of any top companies, MS RA has an added advantage in that.

For previous year GATE cutoff please visit the link below for a more clear idea

https://engineering.careers360.com/articles/gate-cutoff-for-me

Hope this helps.