The following suggested action may throw the CAA into a bit of tizzy, but it's the only way to start getting their attention.
In the US or Canada, you would file a Service Difficulty Report, detailing time and place, make and model of GPS, etc.
It's a piece of aviation equipment that failed and had the potential to display hazardously misleading information (if you were IFR, it would meet this criteria).
I'd also send a copy of SDR to the GPS manufacturer and note on the letter that you are also sending a copy to the CAA.
This might get someone's attention who can do something about it!