One of the great things about Godot is that the editor itself is a "game" running on top of the Godot game engine! This means that improvements to the Linux port will be noticeable in both published games and the editor itself.
As a consequence of running the editor, Godot also has great support for UI primitives, which I'm taking great advantage of in my own game!
I used Godot to make a tool that was not really a game at all, and I read about others doing that as well. It works great just as a simple WYSIWYG GUI editor and GDScript is not a bad scripting language.
Godot has its own UI/layout system. They offer text labels, controls, anchor points, and other conventions for laying out UI. It's not simply coordinate-based like you might expect with a canvas.
I linked the 3.1 page just because it had some graphics that showed the UI features in action. The 4.x pages lacked that. If you wanted to actually start learning these for development purposes though, then yes use the 4.x pages!
Yes, if you need a quick cross-platform GUI prototype it is very nice. One major downside is, that it will use more resources than necessary when idling.
As a consequence of running the editor, Godot also has great support for UI primitives, which I'm taking great advantage of in my own game!