Interesting! The youngest
Gladys I know is late 20s-early 30s (and is a French-speaking Congolese).
It seems less dated than
Janice (sorry, Queenv!), probably because it's actually more dated. And if it was pretty a hundred or so years ago, why shouldn't it be pretty now? No unfashionable sounds, unlike
Gertrude; no unwanted ambiguity, unlike
Fanny. The only possible problem would be a resurgence of the jokey old nn: Glad-Eyes! But perhaps the world has improved and it won't happen.