Hello,
You can apply for any branch that you like for GATE, irrespective of what you have studied in your Btech. Changing the branch is completely upto you. You can definitely get a job if you are talented, irrespective of what branch you chose. When it comes to whether continuing or changing major for masters, it is you who should be able to make the decision, because you know your strengths, weaknesses, interests. If you are unable to make a decision about it right now, take sometime and learn about what is happening in various fields, and then choose the branch that fascinates you the most.
If you don't like the approach mentioned above, you can pick CSE or ECE, because those branches are evolving at a faster rate in the present day scenario. But still, it is completely based on your interest, passion.