1. Get a .gma extractor program. (I use SharpGMad.[www.facepunch.com]
2. Find "assmod_for_gmod13_wplugins.gma" in "GarrysMod/garrysmod/addons" folder.
3. Extract everything and place the stuff where you want.
4. Open .lua files you want to edit with a text editor (++Notepad[notepad-plus-plus.org] recommended).
5. Save the edited .lua.
6. Create a new folder and import everything you extracted from the .gma including the edited .lua files.
7. Name the folder whatever you want and place it in "GarrysMod/garrysmod/addons"
8. Launch Garry's Mod and disable/unsubscribe from this addon. Because if you don't it will conflict with the edited version and the subscribed version will have priority over the edited version.

Hopefully this will help , with this method I've managed to change my MOTD, disable the grammar, disabled the "kill" block command and more.

If you have any questions, ask. Also please note I am NOT a .lua editor.

Correct me if I'm wrong.