GATE

For preparing for GATE you have to buy all subject's books which ask in GATE and then after solving all those subjects you should join test series to analyse your self. You can follow this procedure. You Have to give very much time to your study you can't even skip a day . For gate you have to prepare more .
Good luck.

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Go for made Easy Publication.
And for study material, go for GATEFORUM online series or mock test. It will help you.
Best of luck.

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.