Garling is prideful, and likely think no one can fool him
Also prideful about his bloodline and the strength of his family compared to the other HK/CD families
Shanks was stronk and worthy, Garling would have ditched him asap if he was weak. He likely think the kid being strong meant it was thanks to him/DNA and he can model him to be like him
And frankly like all CD/HK I think the guy can’t intellectually understand that a noble would prefer to mingle with the lowly commoners instead of being part of them.