Notes about the GUI file extension:
Incorrect file associations could be the result of underlying issues within your PC system. As such it is highly recommended to scan your PC for invalid registry entries, unused processes and other unfavourable system settings to identify slowdown issues.
The GUI file type is primarily associated with 'Doom' by Id Software, Inc.. Doom is a computer game by originally written by id Software in 1993. It is considered a landmark title in the first-person shooter genre. It is widely recognized for its pioneering use of immersive 3D graphics, networked multiplayer gaming on the PC platform, and the support for players to create custom expansions (WADs). This file provides menus and HUD in Doom 3 engine-based games.