Firstly, only a fool will blindly follow a GPS and its controlled airspace depictions. But for some reason the anti-GPS traditionalists always throw this one in. It's a basic lack of understanding, usually based on never having used one.
Secondly, yes reception will be marginal inside a metal cockpit. That's why a rooftop aerial, or one clipped to the top of a window, is desirable. There is no such thing as GPS losing reception in a particular area.
Thirdly, the Ontrack report (not IMV terribly professionally produced piece of statistics but people like to quote it) shows that only about 18% of CAS busts were done by pilots using a moving map GPS. Of the pilots doing real flying from A to B, vastly more than 18% use a moving map GPS, and practically none of them use a non-moving-map GPS (which is near useless as it doesn't deliver situational awareness). The only way you will bust CAS with a GPS is if your brain goes to sleep for a while (which does happen sometimes) or if you made a gross planning error back on the ground (which one would have made with or without a GPS).
This is why GPS usage should be in the PPL as standard.