I believe the situation is as follows:
Firstly I assume you are referring to the IFR diversion leg of the nav where you have to fix your position.
As GPS may not be used as the primary means of navigation when IFR - there's a pink AIC about this - you must get you position fix from classic aids such as VDF, ADF, VOR and DME. It is only when these are not available due to range, low level, maintenance periods, etc that the GPS can be used. When GPS is used you may only used raw data to determine your position. You may not use Lat/Long as that is your position! Nor can you sneak a peek at the map display! Raw data is taken to be a plot of range and bearing from a selected waypoint.
At other times, when you are not in the navigation part of the test, you can use the GPS in any way you like. A good example would be in the GH section if an emergency was simulated where an emergency diversion to the nearest airfield was required. Here the GPS can help you determine the closest airfield, help you track to it and even tell you the R/T frequencies you need. If it helps expedite the diversion it is good airmanship to use all the tools at your disposal.
Lastly, don't take my word for it. Why not telephone the CAA examiners and ask. After all, we pay the CAA enough in fees so why not use the service? I have always found them to be friendly and approachable.
Good luck with your CPL skill test!!