I would pick a middle name that is a little longer since Gwen is so short (unless you are using Gwyneth or Gwendolyn). ie. Gwen Elizabeth
How about Gwen Arabelle? Gwen is sort of a romantic sounding name, Arabelle would be a nice fit...
How about drawing from family names? Your name, grandmas, etc.
Gwen Amelia, Gwen Abigail, Gwen Sophia, Gwen Lucia, Gwen Amanda, Gwen Annabelle