I'm not a fan of the flow either, generally they say a long FN should be paired with a short and sweet MN but with this one I disagree. I feel like it needs an equally beautiful, flowing, polysyllable MN to go with it.
I really like PP's suggestion of Elisabeth, spelled just like that too. My MN is spelled Elizabeth but I liek the way the s version looks when combined with Gwendolyn.
Evangeline is also a beautiful name but maybe a bit much with Gwendolyn, with Gwyneth I like it but I like the name Gwendolyn much better!
Evelyn (although they have the same ending so maybe this one isn't so good lol)
Emma/Esme/Emme/Emily all are beautiful