Not sure of any GUI standards, but I would start my search at w3c.org. I mean if there was a GUI standard for the web then this place would have it.
I'm sure, other than that, you will find a host of material on google surrounding best practices, but, really, a lot of the information is fairly logical and more common sense than anything. There isn't, really, a standardized GUI guideline for the web, that I'm aware of, since you can make the site look as ugly as you want and navigate as deeply as you like too, just don't do it.
Maybe you could precise your question. Personnally I would mostly relate the "GUI" label to a program executing directly on the OS (Windows, MacOS. Linux) where you can have use the OS standards or have your owns standards for UI. Web based application executes in a browser that controls a large part of the user interaction (close, back, button size). So you can have GUI standards, WUI standards, and standards/recommandation to test both of these worlds.