So Garling was the boss of God Valley at the time, naturally he would fight with Garp and Roger against Xebec
Somehow after the battle baby Shanks ends up with Roger

-Did Roger kidnap Shanks while Garp and Garling were engaged with Xebec
-Was Garling knocked out cold and couldn't retrieve his son
-Was it Shanks' mother who decided to leave her son for the wolves
-Did Shanks have a birth defect that deemed him incompetent for the mighty respectful Figarland family
-Did Garling at any point try to negotiate/ battle Roger for his son
Too many questions
Somehow after the battle baby Shanks ends up with Roger

-Did Roger kidnap Shanks while Garp and Garling were engaged with Xebec
-Was Garling knocked out cold and couldn't retrieve his son
-Was it Shanks' mother who decided to leave her son for the wolves
-Did Shanks have a birth defect that deemed him incompetent for the mighty respectful Figarland family
-Did Garling at any point try to negotiate/ battle Roger for his son
Too many questions