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The phrase the Doctor used in "Journey's End" was "spatial genetic multiplicity" — intended to indicate that they looked alike without in fact being related.

Of course they're both played by Eve Myles, and "Journey's End" mentions that her family has lived in Cardiff for a very long time, which could imply that they are in fact related. But Russell T Davies has said that they aren't related, and Steven Moffat said, "They don't have anything other than that both their names start with 'G'."

That said, as no one has explicitly stated on screen that they are not related (the bit of technobabble by the Doctor doesn't count as he's just guessing), there is nothing stopping a future Doctor Who or Torchwood story (TV, audio or novel) from establishing that they are, in fact, related.