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It’s funny because GNOME was the first OSS X11 desktop environment to get actual usability testing from corporate developers (Sun Microsystems).
I’m not sure if they still have a user interface design guideline document, though. They probably burned it when GNOME 3 development started. Haven’t checked. I’ve mostly used Xfce since then (and very recently KDE).
https://developer.gnome.org/hig/