GATE

I think you should get around 75-80 GATE score in order to get your desired seat in the desired college which is IIT Kharagpur for CSE. So, you have to work hard. Good luck!

M.Tech seats are filled only through GATE exam.

No, you cannot appear for BCA after GATE.

If you feel like working for core companies and have a desire to work for a better organization rather than crawling in those MNCs, then drop a year and do preparation for GATE.

This is according to the GATE 2017, negative marking will be only for the multiple choice questions (MCQs). For 1-mark multiple choice questions, 1/3 mark will be deducted for a wrong answer. Likewise, for 2-mark multiple choice questions, 2/3 mark will be deducted for a wrong answer. There will be no negative marking for a wrong answer in numerical answer type questions.

Most institutes state minimum eligibility to be an undergraduate B.Tech degree in Chemical Engineering. - The Indian Institutes of Technology have the added eligibility criteria of a valid GATE score, which is only relaxed for those who have obtained their B.Tech from an IIT with a CGPA of more than 8.0/10 as well.
Note- Some institutes allow the equivalent forms of B.Tech in Chemical Engineering, such as a B.E. in the same field.
Regards.

Preparing for GATE is a good option. It all depends on your interest whether you are willing to pursue BE or B.Sc.