The fact that Sommer as a holy knight didn't age since GV when Garling did suggest not all holy knights have that contract with Imu.
Either Imu decides who to give it to, or its a contract you have to ask Imu to give you, and thus most likely Garling never really accepted that contract even as the leader of the holy Knight
Maybe he didn't want to be bound by him judging by what happens to those who betray him or he thought it wasnt the right time(depending on if he accepted it as a gorosei member now or not) or he was simply never interested in the immortality thing, likely aiming for something else
Either Imu decides who to give it to, or its a contract you have to ask Imu to give you, and thus most likely Garling never really accepted that contract even as the leader of the holy Knight
Maybe he didn't want to be bound by him judging by what happens to those who betray him or he thought it wasnt the right time(depending on if he accepted it as a gorosei member now or not) or he was simply never interested in the immortality thing, likely aiming for something else