Switching field of study from computer science to GIS in the post graduate level
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".