I'm wondering after reading the official scans, that once Imu leaves Marijoa, the Perpetual Youth Surgery ends.
This would explain why the other Gorosei were freaking out, but not Garling.
The other four Gorosei have their souls tied to Imu, and are losing their perpetual youth due to him descending to the lower world.
If this is the case, this is very interesting for Law, because it would mean that they would then need to reaquire his fruit in order to regain their perpetual youth.
Garling never got this operation in the first place, so it's in his best benefit for Imu to do this. Now, Imu and the Gorosei NEED to get Law's fruit back, and this leads Garling to also getting the perennial youth surgery.
Big brain plays.
This man is a monster.