Therein lies the contradiction - that Gaeliss cannot pre-exist herself in her own 1940-1968 timeline. To do so, is a metaphysical impossibility.
I recommend reading "The Gabaldon Theory of Time Travel" in The Outlandish Companion, as well as the "Note on the Genealogy of Roger MacKenzie (Wakefield)". (The Companion was published before The Fiery Cross, so there shouldn't be any spoilers.) Diana explains her rules for time travel better than I ever could.
The premise that Geillis "cannot pre-exist herself" is not one that the author appears to hold true, and for me, it's the author's prerogative to establish those rules in a fictional universe. Diana sets certain limitations – see the Companion section on non-simultaneity – but the text is consistent on there being one reality in which all of the characters exist.