i think you can opt for aerospace engg,cos aerospace engg has a lot on electronics side like avionics,guidance and control etc, but which paper you have to take in gate i.e., whether electronics or aero depends more on the institute you apply.its always better you find out what are the specilisations the colleges like iits offer in aero and whethet it is related to ece and its better you call the colleges and find out which paper you need to take in gate to secure in seat in aero , given your background in electronics. i would advice if you want to prepare for aerospace engg paper in gate it will really tough since you are from ece and as for as i know there is no coaching center for aero ( cos i did b,tech aero and am actually lookin for  study material) so you can choose ece in gate and opt for course which is  specialization in aero related to eletronics.