hi,

see robotics is an interdisciplinary stream , you may become a robotics engineer by studying in cse stream or embedded stream or instrumentation stream. from this year candidates are eligible to apply for two gate paper, so you may apply gate from mechanical engineering or computer science engineering. however now it depends on you whether you can take preparation for two subjects simultaneously. however my suggestion is to take mechanical paper of gate, since you have studied it in b.tech and have better understanding.

hope it would help.