Actually you have a pretty good chance of getting admission in IIT-B if you are also applying for TA position you look at the last year's rank. The GATE score of the last candidate (general, TA category) admitted to IIT-B in Energy Science and Engineering is 679. However, the GATE score of last candidate to get admission in the same department in 2017-18 session is 723 (general). As you are from Mechanical branch, the cut off is higher for getting admission into another branch.