GATE

Yes, of course, but you have to prepare from the syllabus of aerospace engineering only. The B.Tech syllabus of aerospace engineering will be given in the test of GATE exam. You can join the coaching since you are shifting the branch which may lead to face some difficulty. Don't worry focus, work hard and study well. You will definitely get the college through GATE.

Yes, you may get any autonomous college for chemical engineering with this score.

You have so much time for GATE exam. Even if you want to give it when you are in the 3rd year of your graduation then also you have 2 years. Download the syllabus of GATE for ECE students and start studying and for help join some online coaching like that of Ravindra Babu. It will provide you with the study material along with the necessary guidance.

You have to get rank under 100 or 200 hundred to get any IISC through GATE.

You can start preparing from today itself. If you have ample amount of time join a coaching class near your residence or else go for online course. There are many tutors like for eg. Ravindra Babu. His videos are really helpful.

Please check for your eligibility to apply for the GATE exam from the criteria mentioned at the link provided below:
https://www.shiksha.com/engineering/exams/gate.

Hi Mayank,
BITS HD Entrance Exam is similar to GATE examination. If you are clear with your B.Tech subjects, and in touch with the concepts, you will do well. Preparation of GATE, gives you an upper edge in BITS HD Entrance exam, as the questions are not very different. All the best.