@Ace Bng garling being Shanks' father is just a fan-made theory. A theory I have heard now is that, the silhouette shown under dragon is His evil twin(and he's the one who was talking to gorosei during wano arc).
@AJ NAIR Like 99% of One Piece theories it's just a wild guess so until Oda sensei says otherwise Shanks doesn't have a brother. For Garling just like you said he is supposed to be Shanks' father so it's just an hypothese that comes from OP Red movie