Gilly, as we know, was taught to read by precious cinnamon roll Shireen Baratheon (SOB), and has been excitedly devouring books in the Citadel while Sam has been shovelling shit.
She's come a long way, but she still isn't great at the pronunciation of certain words – which is especially understandable when it comes to Targaryen names. So when she says "Ragger", she is 100% talking about Prince Rhaegar Targaryen.
You know, the Rhaeger Targaryen who is most definitely Jon Snow's father. Who supposedly kidnapped Jon's mother, Lyanna Stark, but who many fans have long believed actually eloped with her.
Gilly's little discovery confirms the long-held fan theory that Rhaegar had his first marriage to Elia Martell annulled, and then married Lyanna Stark.
Which means not only is Jon Snow NOT a bastard – but also that HE IS THE TRUEBORN SON OF RHAEGAR TARGARYEN, AKA NEXT IN LINE TO THE TARGARYEN THRONE.
How appropriate that he got up close and personal with a dragon for the first time in this episode.