Gladius Mods are just standard UE4 plugins that we build and configure in a special way so they can communicate with the Game.
Open the UE4 GladiusModPack project and go to Edit > Plugins and click "New Plugin"
Select "Content only", type the name of the plugin (e.g. MyMod) and then Create Plugin.
This will create a new folder in the content browser:
All Gladius mods need to have a custom definition file at the root of the plugin content folder, that just tells the game the type of mod and the main file to open. For example in the Infinity Arena example: