I feel that Sentinel's strength fits Galio well as a protector giving you a bonus when your allies are in danger to help save them meaning in team fights you will have a bigger presence with your allies taking allot of damage.
now with a more defensive use to it thank to Sentinel's strength Resolute Smite will in turn have weaker damage early game but will have more damage potential late game thanks to a lower cooldown when it is maxed.
making Galios Bulwark a shield gives it more visibility to allies and enemys making it easier to play around and reward breaking it, while still having a heal that is similar to the old Bulwark in that your taking damage to heal.
With a higher cooldown and it having Righteous gust will be used more as a engage and disengage tool than a damage tool . The directional draft left by Righteous gusts has double the width than it did so your allies will be able see and use it more effectively to engage or disengage.
Not much changed here just a lower cooldown and lower damage .