At the end of the third season Bjorn was taking Ragnar home and wanted warriors to stay behind to keep a base for the next raiding trip. Rollo volunteered. While there, Charles offered one heck of a bribe making Rollo Duke of Normandy (will be later in history) and giving him Gisla in marriage in return for defending Paris. Rollo saw it as fulfilling the Seer's prophecy, I guess.
Rollo's story is one I'm really looking forward to this season. Both his transformation from viking berserker to somewhat Frankish nobleman as well as his relationship with Gisla, Gisela is the proper name I found online. Her existence is disputed, though. Rollo is credited with another Frankish woman Poppa who his kids are descended from. Who knows if they'll give us that second wife or combine the two women in Gisla? I expect combining them, starting with the troubled marriage alleged with Gisela, and then the happier one with Poppa. The show has already combined one wife in Judith. Aethulwulf's first wife was Osbuhr and she was the mother of Alfred. Judith was a Carolingian princess he married later in life. So, it wouldn't surprise me if they did something similar with Rollo's wives.
