Gladys / Gwladys is like the quintessential old-lady Welsh name. There were just so many of them in the generation that is now over-60 that I can't see it coming back here for a while. I like it though, it's a happy-sounding name.
I'm not sure how much
Gwendolen is used here these days, but I've never met or come across one.
Gwenllian seems to be the top choice for a long version of
Gwen in Wales right now. It reminds me of the girl in
Daniel Deronda.