If you are talking exclusively about GATE, remember, the results depend upon your rank.
If you were to opt for CS (general category) and have a rank below 150, your chances of admission are high. This may be the same for other branches as well.
Ranks vary with the specialization you opt for. For instance, when it comes to electrical, VLSI is one such specialization that fills up before others. For VLSI, your rank should be close to 100.
As far as CS is concerned, you do not need a specific specialization. No interview would be conducted and your admission would depend upon one factor- your GATE score. For MS, there would be a proper entrance followed by an interview and its eligibility depends upon your GATE score exclusively.
Admission Procedure: All IITs conduct their own admission and almost everyone applies there. Because of this, not one, but 4-5 lists are published by IIT-M because of the number of varying offers.
Spot admissions also take place in the case of seats that remain unfilled. All the information about these spot rounds would be disclosed on the website.