"As Geb, cleanse a teammate using Stone Shield on an ally that is below 20% Health."
You need to use Geb's third abillity Stone Shield
on an ally that is being affected by any crowd control effects AND
that is below 20% health.
Your best shot here is to play Arena matches and keep an eye out for your allies with low health. You can check all the crowd control effects and the gods which inflicts them here:http://smite.gamepedia.com/Crowd_Control