Grade B-C (depends on version)
Press the ~ key to open the console.
To disable the hud, enter: toggle g_showhud
To disable the gun, enter: toggle ui_showgun
If you’re playing Doom 3 BFG edition, those commands won’t work until you do the following:
Create a shortcut to Doom 3 and add these commands to the end of the target line:
"Doom 3 BFG\Doom3BFG.exe" +seta com_allowconsole 1 +set g_fov 90 +set com_skipintrovideos 1
Only the first command, +seta com_allowconsole 1, is necessary, but I recommend setting the fov to 90 and skipping the intro movies as well.
Now you can start the game and enter the aforementioned console commands.
If you want to press a key to enable/disable the hud at will, open the console and type:
bind h toggle g_showhud
Replace ‘h’ with the key of your choice. Unless you turn the hud back on, it should stay off.
Remember to always start the game with the shortcut.