R. Jeeves: In my experience, ladies who spell Gladys with a W are seldom noted for their reliability, sir. It gives them romantic notions. B.W. Wooster: With a W, Jeeves? No, no, no, no. You spell it with a G. R. Jeeves: If I might draw your attention to the signature on the portrait, sir. B.W. Wooster: Good Lord! G-W? R. Jeeves: I blame Alfred Lord Tennyson and his Idylls of the King. It also accounts for Kathryn, Ysabel, and Ethyl, all spelt with a Y, but Gwladys is a particularly virulent form, sir.
Source: wiktionary