Where should i create my combat related functions in a RPG+Multiplayer game?

Hi there, i am trying to make an RPG game that will feature multiplayer. I am at a point when i want to implement functions that will drive the combat mechanics, such as Damage, Armor, Resistances, That type of things.

My question is, where should i implement that? I was thinking of game instance or mb gamemode? but i dont really know.

These functions should only be available for the server. The server will be doing all this math to prevent players from cheating. I will use them in many other blueprints too.

Ty in advance and sorry for my bad english.