The Gwendoline form may introduce pronunciation confusion -- does that last syllable rhyme with wine or win or when? We vote Gwendolen as not only the most proper but the clearest spelling, followed by Gwendolyn, with Gwendoline a distant third.
'By far the most popular form in Britain of GWENDOLEN, at a peak in the 1920s but now quietly used.'
Gwendoline (1886), opera by Emmanuel Chabrier
Gwendoline, character in Céline's influential novel Mort à crédit (1936); voracious sexual partner to Ferdinand (the narrator) on channel crossing
Sweet Gwendoline, character created by bondage artist JohnWillie; played by Tawny Kitaen in the sexploitation/adventure film Gwendoline (1984)
Gwendoline Mary, character in Enid Blyton's book series 'Malory Towers'