Simply put, one should be able to create a custom gamerule category, then move existing vanilla gamerules into that category. E.g. One can create a DAMAGE category and move GameRules.FREEZE_DAMAGE into the new DAMAGE category.

This is another one of those changes that requires modification of enum constants.
The game rule categories are an enum.
See:
FabricMC/fabric-loom#305
SpongePowered/Mixin#387