The core of the modding system uses .gd files in the mod as though the mod folder was the "files" folder to apply changes to matching .gd files in the game folder. To do anything outside of that core you need to use either the "patch" folder or the dynamic mod folder functions, both of which are explained in the modding guild found in-game in the Mod menu's "Help". Note that you need to use the BugFix mod if you want to use the patch system to add new folders for the new files.
English is not Mav's first language, so he missed some of the subtle parts of the language.