GPS should be a lot more accurate than radar. It's usually within a few metres. Commercial radar won't give you that at say 20nm.
The CAS shapes in the GPS database normally come from the national AIP of the country concerned, where they are given as lat/long coordinates, arcs etc and that data goes straight into the GPS database. Sometimes these are wrong, but it's very very rare. I've never heard of this in the UK, in 6 years I've been flying. But obviously where CAS has changed then unless one has updated the database.....
It's not a good idea to use GPS as the sole CAS shape reference, not because it's wrong but because it's not very clear. If one could get a large panel mounted GPS that displays (in the UK, anyway) the actual CAA charts, that would be great, but one can't buy one.