In the context of Roblox scripting or "exploits," a GUI refers to a custom window injected into the game to automate tasks. These typically offer:
Instant movement to specific bosses like the Orc Boss , Wizard , or Pirate Boss . Community Perspectives
“The GUI is the main game HUD... it displays all the information you need in BFS.”
Automatically swings weapons or attacks bosses to gain power without manual clicking.