Hello 

As you have done BE in Mechanical and wants to give Computer Science paper I wont think so it will be possible because u know right Mechanical is completely a Machines thing and CS is Software related if you are really intrested in Programmes then why cant you opt for robotics or mechatronics by writing GATE in Mechanical itself, Gate is completely a core exam so try in your branch itself to be successfull. Hope you have got an answer 

Thank You