You have to apply for Computer Science and Information Technology in GATE.

And try to get Good rank because it will not only help you to get in good college but also let them believe that you can do Masters In CSE even though you have done your Btech in chemical Engineering.

pls ignore my English Grammar

Hello,

You can't do it like that. You can't change your stream in higher studies. Higher studies means specialization in one domain from your stream. You can't change your stream.

In gate you have to apply for Chemical engineering only as you have studied it in your under graduate also.

You have to prepare for the papers from your branch only and they will cover the subjects from your degree course only. You can get the complete syllabus from here:- https://engineering.careers360.com/articles/gate-syllabus-for-chemical-engineering

Hope it helps!