If it was fog or low cloud no amount of home made GPS approach accuracy would make any difference!
It would not matter if there was an ILS on the field unless the pilot was prepared to go below minima.
No pilot should use GPS on its own! In my piston twin days I used to operate into an airfield with terrain around. There was an NDB and a DME but no approved approach.
I overlayed a GPS approach over the NDB/DME used the GPS for precision and the NDB/DME as well a a RadAlt to confirm the GPS.
Just to use GPS would be very foolhardy.
More likely with areas being in the clear that the pilots were flying in visually and seeing cloud ahead made some mistake on their location especially if you think you know the area well.
That fact alone will make a pilot less cautious but more prone to a serious mistake.
Hence why Graham Hills crash came to mind as he misidentified visually albeit at night where he was and continued beleving he knew where he was on an approach he had made many times.
Pace