Specifically, how do you see GPS being examined?
Same as any other navaid receiver.
But there is a different take on all this: do you want to examine competency in the abstract, or do you want to examine competency for the actual job as it is done in reality and as it would be done in a realistic emergency procedure?
The present system does the former. If your vac pump packs up and you go partial panel, you don't tell ATC you have a problem (because you are a proper JAA licensed hero and this character-forming event will only help to grow the proper JAA certified hairs on your chest) and you bust your gut trying to make the best of a bad job.
In the latter case, you tell ATC, tell them you are partial panel, that your responses will be less timely, that you need either vectors, or time to set up your handheld GPS, etc. They are paid to assist you and only a fool would do less than take full advantage of this. And if you have a total loss of electrics then you better have a handheld GPS all set up already because the ICAO procedure is to fly the filed route (after 7 minutes on current heading if in UK airspace and on vectors, or whatever) all the way to the end, and turn up at the IAF at the filed time, and land.