You can take the different stream in GATE entrance exam . It is just that if you want to apply for a particular stream , for that stream you should have a valid gate score . Some colleges allow to change the branch if you have done BTECH. from a different stream  . Most of the IIT'S have a criteria  that if you want to change a branch in Mtech than it should have some relativity for the branch you have studied in Btech .  You have studied automation and robotics  , so if you want now computer science or information security , so you can apply for Mtech in computer science engineering because it has relativity to your automation and robotics .