For Nippopolis is a side mission in the video game, South Park: The Fractured But Whole.
Plot details follow.
Bring Gerald a Cheesing Vial.
After unlocking the Fartkour Buddy Power, go to SoDoSoPa. On top of the rundown building, you will see a pinwheel. Use inspection mode on this object in order to Fartkour to the top.
Here, you will find Gerald. In order to bring him back to reality, you will need to offer a Cheesing Vial. There is a high chance that you will stumble upon one in the first day. If you do not already have one, simply head down, and obtain a vial from the red car in SoDoSoPa. You can also go to Kyle's House and find one upstairs in the attic.
Give him a Cheesing Vial to finish the quest and unlock the Cheesing Gerald summon to use in battle.