Russell's hint in DWM suggests Captain Jack's partner will be named Gwen. It does seem to be a rather interesting choice for a character name. This name could either be a shortened version of the Welsh names Gwenhwyvar, Guinevere or Gwyneth or of the Celtic Gwendolyn.
If you are looking for clues, Gwenhwyvar translates to "The White Shadow," Guinevere slept around on King Arthur with Lancelot, and Gwendolyn is the name of a shareware program for finding name origins! The word "white" came up quite a lot in my name research, but that doesn't seem to give us much to sink our teeth into.
Nope, I'm convinced that searching in the name origins arena is a wild goose chase. What has popped out at me, is that one of the most famous Gwens around is the transgendered Gwen Araujo, So that clinches it. When the BBC reveals that Captain Jack's partner in Torchwood will be a she-male don't forget that you heard it here first!