M.Tech

Hello Ashwini,
I am afraid, you won't be eligible for PG in aeronautics after graduation in electronics. But why do PG in Aeronautical engineering? Aerospace industry needs experts in different field including electronics. You can get job in aerospace industry based on your electronics qualification also.

You can choose any course as per your interest and study. HR is mainly for girls and has scope for them so Supply Chain Management is an better option but MBA should be done from institute of good repute so that you can get a good career start.

Yes, GATE exam is compulsory for all the students who wants to go for higher studies after B.Tech and through GATE exam score, you can get the college.

Basically, Fee for M.Tech in CS branch is different in different collages.

Please refer the below link.
https://www.google.co.in/url?sa=t&source=web&rct=j&url=https://www.shiksha.com/engineering/course/amp/m-tech-in-civil-engineering-part-time-lovely-professional-university-admission-office-connaught-place-delhi-229120&ved=0ahUKEwjVlsW-yrHWAhXEO48KHSjJDD0QFghsMAc&usg=AFQjCNGbmvl3FifBi-pTLV9wIh3cNBMKDQ&cf=1

Hardly any importance is given to M.Tech through Distance education. Try full time or evening/week end classes.