Bonus MS towards nearby allies that are movement impaired or enemies that you impair movement on.
Aftershock is a fantastic keystone choice for Gragas because it provides him with the defenses needed in an engage, as well as a bit of extra offensive power with the explosion.
WhyFont of Life
Gragas can very easily apply Font of Life to a large number of targets from his [Q], [E], and [R] results in a lot of healing in teamfights.
Gragas naturally builds a good amount of health and that health is multiplicative with every defensive stat he gets from Conditioning.
Revitalize synergizes incredibly well increasing the healing of both Gragas' passive and Font of Life healing.
The one-time stasis active from Stopwatch is pivotal in executing early tower dives that can grant your team a snowball lead.
Approach Velocity on Gragas allows you to stick to a target for continuous auto-attacks after landing a [Q] or [E]. The extra Movement Speed is also great for allowing you you to get just far enough ahead of your opponent to land an [R] past them that will knock them back into you.