Switching field of study from computer science to GIS in the post graduate level I have a B.Sc. in CS, but I want to further in the GIS (geographical information system) line. I am thinking I'd have to do some sort of post-graduate diploma in GIS, before I can go for a M.Sc., which is necessary to hit my mark, seemingly...

I need your guidance about this case.

Thank you very much
I can't comment with authority on your specific case, but some GIS masters courses may not assume much GIS-specific prior knowledge. You should probably have an interest in mapping, cartography or spatial data, and - since you are coming from a CS direction - in databases or spatial computing; but interest does not necessarily mean in-depth knowledge. 

I think my advice would be "find a course that interests you, and discuss it with the person in charge of admissions".