The approval you get from the GPS course relates to IFR anyway, so there really is no advantage in terms of a VFR flight test, ie you don't need any approval to use the equipment as supplemental means.
The CPL test form says at least one leg requires basic visual navigation, so the ATO will not allow you to use it (or any other navaid), probably when you are on the leg that requires you to locate a place remote from significant ground features.
Having said that, if you use the GPS when the ATO allows, make sure you do what it says in C8.8 of the syllabus. You have to verify the integrity of the signal and check the waypoint information against another verifiable source.
I think as a general rule if equipment is installed in an aeroplane then the pilot should know how to use it properly, so i applaud your decision to give it a go in the test. It's amazing how many people refuse to use autopilots and GPS despite the pre-flight briefing including "feel free to use all the equipment on board - let me worry about the test requirements, you just do what you'd do if this was a charter flight and you were the PIC..."
Sorry for the thread drift, i'm not in ML so don't know any places to do the course. Good luck with the test.