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.
Gwendoline Christie, English actress
(Gwendoline Maud) Syrie Barnardo, English interior decorator
Gwendoline ElizabethDavies, Welsh philanthropist and art collector
Gwendoline Didier, French figure skater
Gwendoline Butler, English mystery novelist
Gwendoline "Gwen" Harwood, Australian poet
Gwendoline AlicePorter, British Olympic runner
Gwendoline "Gwen" Raverat, English artist and writer, granddaughter of CharlesDarwin
Gwendoline GaelleRamos Ruais, Miss World Philippines 2011
Gwendoline See-Hian Yeo, Singaporean-born American actress
Gwendoline (1886), opera by Emmanuel Chabrier
Gwendoline, character in Céline's "Mort à crédit" (1936); voracious sexual partner to Ferdinand (the narrator) on channel crossing
Sweet Gwendoline, character created by bondage artist JohnWillie and 1984 film "Gwendoline"
Gwendoline Mary, character in Enid Blyton's book series 'Malory Towers'
'By far the most popular form in Britain of GWENDOLEN, at a peak in the 1920s but now quietly used.'