Short answer: You can’t use username or password without breaking GL GUI.
Long answer: AGH is integrated into the login procedure by GL GUI. This happens during startup of the AGH instance - because it will load a settings file created by GL. You can, of course, modify the startup procedure, so AGH will act like a standalone installation, but in that case, you can’t configure AGH by GL GUI anymore. This has advantages and disadvantages, and I would not really recommend it for non-professional users. Some more information could be found here, even if it’s just about the updates.