Hello Ravi dubey,
I would like to infrom you that, you should not change your subject in M tech, it also depend upon that Gate exam based on your interest so you can change your subjects in whch you have done engineering.
Best of luck
Yes you can do.
But it will be very difficult to concentrate .
Many students of ECE branch prepare to get into cse in Mtech. Generally it's tough but not impossible you should devote lot of time in preparation of gate . There are many in good courses online learn competitive coding try to purchase online learning courses
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