How much score should I get in GATE to secure a seat in IIT
It depends on the difficulty of the paper, and the IIT in question. Some IITs may require further qualifications (like IIT Delhi requires an 8 point CGPA in addition to a good GATE score). Others may have interviews for some candidates (but this is mainly for the 3-year MTech program).
On the whole, you require at least 60 to get a direct admit in any IIT (among the old ones), and more than 65 to get an admit call from the IIT of your choice.