Moddable

This utility can easily open the compressed game files that were installed with the game so that you can gain access to all the moddable game files […]

To make things even more confusing, Garry's Mod is a "moddable" mod, meaning that users create modifications that plug into Garry's Mod itself.

Strip off that veneer and a whole world comes into view, one that's eminently hackable, moddable, tweakable, customizable, and personalizable.