I don't think there really is a mono version of the Kontakt player...there are no mono versions for Kontakt or any other Kontakt/Kompakt/Intakt-based libraries. The fact that mono appears as a choice in the instrument plug-in menu is more likely a bug in NI's Audio Unit installer for GPO. Maybe as simple as some box that shouldn't have been checked when they compiled the plugin.
I don't think you can assume that a mono GPO, if it even existed, would actually be any lighter in processor load. The instruments in GPO ARE mono, they just get panned across the stereo ouputs. Use the stereo version and don't worry about it.
I'll pass a note on to NI about this (I'm a beta-tester).