Mad Jock's reply was, in fact, quite correct, if he meant something other by 'GNS'.
There are a very significant number of commercial types which prioritise rho-rho or rho-theta over GPS. It is only recently (the last decade, off the top of my head) that some airliners prioritise GPS for the navigation solution, and even this is with a 'confidence check' from available ground stations (the manufacturers are known to be guarded about precisely how the navigation solution is derived in these situations).
Although MJ may have meant something by 'GNS' other than that which you inferred, Peter, you should not make bold statements like that.
You're too often wrong.