My Welsh grandmother's name was Gwladys, which seems to be a variant of this using the more common Y from Gladys. The name of the Roman Catholic saint Gwladys (who was a queen of Gwynllyg in South Wales) seems to vary between the U and the Y spelling depending on the source. There is a French golfer who also uses the Y spelling. http://en.wikipedia.org/wiki/Gwladys_Nocera I would hazard they are interchangeable, but that the Y spelling is more modern.
The name of the Roman Catholic saint Gwladys (who was a queen of Gwynllyg in South Wales) seems to vary between the U and the Y spelling depending on the source.
There is a French golfer who also uses the Y spelling.
http://en.wikipedia.org/wiki/Gwladys_Nocera
I would hazard they are interchangeable, but that the Y spelling is more modern.