If you load the map and see purple walls or giant "ERROR" signs, you likely need the . Most GMod maps utilize assets from CSS. Since you are not using Steam, you must download the "CSS Content Addon" from a site like GModContent and place it in your addons folder just like the map.
While more focused on total conversions, ModDB hosts several GMod map packs that include the original Pizzeria. "FNAF Map Pack Garry's Mod." How to Install FNAF 1 Map Without Steam fnaf 1 map gmod download no steam better
Flickering hallway lights and "Power Out" sequences. If you load the map and see purple