You can write & execute Lua code with the Luapad. Just type luapad into your console to open it.

It supports tabs, saving and opening files, indentation, and of course, running Lua. Note that you cannot use the Luapad to run code server-side. (only admins can.) This is illegal and you will face righteous slaughter if you somehow run code on the server.

Additionally, sv_allowcslua is set to 1, so you are free to load your own clientside scripts as you please. Make an aimbot! Find people with ESP! Create a spreadsheet! Crash your own game! The possibilities are endless and it's all up to you!

Helpful Resources

Here are some tutorials, resources and documentation about Garry's Mod and its Lua API.

• https://wiki.facepunch.com/gmod/ The official garry's mod wiki, maintained by the community and Facepunch.
• https://stackoverflow.com/questions/tagged/lua Every programmer has gone here at one point or another.
• https://www.lua.org/docs.html Read technical information about Lua.
• https://maurits.tv/data/garrysmod/wiki/ An old, outdated wiki about Lua in Garry's Mod. It's still a little helpful.
(R.I.P forum.facepunch.com)