Hey,

Ofcourse it is possible to take Mtech in Aerospace Engineering after completing BTech in Aeronautical engineering. You can write the GATE exam for the branch that you want to pursue. However the eligibility criteria differs for different colleges. If you are planning to do your MTech in Aerospace Engineering, then check this link given below to see the top colleges offering the degree.

https://engineering.careers360.com/colleges/list-of-me-mtech-in-aerospace-engineering-colleges-in-india

Hope this helps.

Thank you and All the best.