Menu plugins were a feature of old garry's mod versions that allowed you to modify the main menu. These were removed in later garry's mod version due to (presumably) security concerns.
This addon will readd these plugins to garry's mod - however use them at your own risk! Only install plugins that you trust!
Simply drag and drop the lua folder into your garrysmod/garrysmod directory. Accept to overwrite any files. (This is required for menu plugins to function)
If you want to write your own menu plugins just look at the default plugins already included here. Especially take a look at the extra functionalty and settings store provided.