Luabee is an ingame Lua editor for Garry's Mod that turns blocks and lines into code. Luabee is open-source, and if you're willing to improve it then please feel free! Don't sell it for money, though. Luabee is free and will always be.
If you want to help add libraries, classes, or hooks, please refer to sh_functionadder.lua for examples and help. It contains a rather large tutorial on catologing functions. It's so easy a monkey could do it. Unfortunately I don't have any monkeys.