Add GMod declarations (#141)

Author: lolleko

docs/getting-started.md

@@ -83,3 +83,4 @@ The real power of this transpiler is usage together with good declarations for t
 - [Typed Factorio](https://github.com/GlassBricks/typed-factorio)
 - [The Binding of Isaac: Rebirth](https://isaacscript.github.io)
 - [Retro Gadget](https://github.com/DarkMio/retro-gadgets-typedefs) ([template](https://github.com/DarkMio/retro-gadgets-template))
+- [Garry's Mod](https://github.com/lolleko/gmod-typescript)