Hi

MSc astrophysics as in is a branch of physics. So out of 29 subjects Gate has to offer you are supposed to go for physics.

Second point is the choice of gate subject should be the one closest to or the same as your undergraduate or BSc or BE specialization rather than the course you are GOING to pursue in future and its also specifically mentioned in GATE notification. If you are going to pick up something else other than what you have studied in UG it will be very Difficult to prepare as the gate syllabus is completely based on the UG or BSc or BE syllabus and to top it all Gate is a difficult exam to crack or score even if you are appearing for your own subject. Though they have given 2 paper options but even then the allowed combinations are fixed and decided other than that  combinations you can't choose anything else. For eg if physics is one paper you have opted for then you can opt for 2nd paper which is only related to physics meaning keeping physics as a main paper you can go for geophysics or electronics. They have also mentioned that the exam centers for the two papers might be different so think ,decide and choose wisely.

I would suggest that you choose your graduation subject and not to make it difficult as it already is moreover its easier to appear for the subject you are already acquainted of or have already studied. Rest I leave the decision to you.

If you have any other query let me know.