She makes connections. Lois. Clem. Rupesh. Rhiannon. Johnson. She keeps everyone connected together, and gets Jack in touch with the people he needs to be in touch with.
She’s always of both worlds. A police constable, but on the run for the law. Torchwood, and ordinary folk. While every other woman in the miniseries is either a mother or not a mother, Gwen is pregnant, partway between both worlds. She can communicate with people from vastly different worlds: Lois, the daring young woman in over her head, and Clem, the old man still stuck in 1965. She can love and obey Jack and still support Clem, the person he hurt. Gwen is even the one to try to call Frobisher in Day Two. Jack sends her to Rhiannon, a woman he’s never met, and probably never will, and Gwen is able to connect the two worlds that Ianto has kept separate with lies and reserve.
There are so many different people in Children of Earth, and Gwen can communicate with just about all of them.