Gaban in this chapter,was i think the third time in the story that he mentioned the concept of destiny regarding Shanks.
Its clear there is some sort of prophecy or something about Shanks. Maybe Roger believed he was Joy Boy,and thats the reason he gave him the Strawhat,altough wee see Shanks wearing the hat even before they went to Laugh Tale. But maybe Roger, Rayleigh and Gaban already knew something about the ancient history at that point