I actually did the same thing, BSc in psychology, and a MSc in Clinical Psychology and I'll be applying for 2014 entry GEM. Obviously when taking my MSc I didn't consider GEM, as the plan was to go toward ClinPsyD, but after a placement on a psychiatric ward a few months ago, it completely shifted my focus and made me much more physiologically and biologically orientated so it doesn't make sense for me to consider ClinPsyD anymore.
An MSc might not (or could - perhaps if you demonstrate it as passion for further learning into a healthcare related speciality in your PS) put you at an advantage in your GEM application, but if you're curious and want to learn more then do it. I'm even researching into neuroscience MScs too because of my curiosity, it's nothing to do with my later GEM application at all.