Neither of them overlaps greatly with gender studies keeping in view the syllabus provided by FPSC. I'd recommend Sociology though as it is really really interesting and does give you a framework to set most of your answers to in gender studies as well.

I appeared in the CSS exams this year and Sociology was one of the papers I enjoyed attempting the most. I had originally picked Gender studies and sociology under the same misguided notion that they'd overlap but they didn't really besides the fact that GS is a social science. I think the reason for this is that the GS syllabus given by FPSC is primarily geared towards the more controversial aspects of the subject and its history rather than the actual theoretical work. The theories and literature of GS would most definitely overlap with sociology since-until very recently-sociology was the subject under which those topics were studied.