In this context, a "GUI" (Graphical User Interface) is a custom menu that appears on a player's screen to provide extra functionality beyond the standard game controls. These scripts are often created by the community to add features like:
Tools used by game moderators to manage players or server settings. City 8 GUI
Custom HUDs (Heads-Up Displays) that match the gritty, industrial aesthetic of City 8. In this context, a "GUI" (Graphical User Interface)