you can opt any stream in gate. But if you want to join PSU then u should write gate with ur gradutaion subject only. If u want to do Masters you can choose any subject in which you like to do

Hello dear,

You can give Gate exam of  that subject in which you have completed your under graduation or doing graduation. If you are from computer science branch then you can give GATE exam in computer science stream. Yes you can qualify gate from CSE branch.

Hope this information clears your confusion.