Given the audience of this site, and given that elementary strives to provide an integrated user interface, I think that a tag for “the graphical user interface” is appropriate.
On a site like Unix & Linux, we'd use different tags depending on whether the issue is with X11, with the window manager, with another component of the desktop environment, with the application, etc. But such granularity wouldn't be appropriate here. So I'm in favor of having a unified tag for “GUI issues”.
“graphical-user-interface” would be the most “proper” tag name, but the GUI is the main interface on elementary OS, so “user-interface” is fine as a tag name. Therefore I propose to keep user-interface in its role, make gui a synonym because it's a common abbreviation, and get rid of ui.