Add the gravity zones mod
Link: https://www.curseforge.com/minecraft/mc-mods/gravity-zones
Description: Gravity Zones is a mod inspired by Robinton's Low Gravity mod and was requested by MineCrak. This mod adds the ability for Custom Gravity Zones to be defined on a server and sent to the client. In addition to modifying gravity you can also modify a lot of other parameters such as Oxygen, Cloud Height, and Sky Color.