"Unlike the Steam Workshop or local content mods, this SDK gives mod makers the ability to change, extend or rewrite TF2, making anything from small tweaks to complete conversions possible," Valve ...

My TF2 HUD and one of many hobby projects. Readme has the info and links you need.

A collection of scripts, tools, and cheats for Garry's Mod to enhance your gameplay experience. Use responsibly and only in appropriate environments (e.g., single-player or private servers).