That depends on what you mean by "system UI".
Most system utilities on Windows are still using GDI. GDI is not GPU accelerated (other than BitBlt).
Same apply to MacOS - CoreGraphics there is pure CPU rasterization library.
And the same situation on Linux/GTK, system uses Cairo - pure CPU rasterization library.
Usually, on modern OSes, window/desktop manager is the only GPU accelerated thing - it composes window bitmaps on screen.
GTK 4 renders by default using your GPU. The only part that is CPU rasterized is if we don't have an accelerated path for it (rare) and glyphs are first rendered on CPU, uploaded to a texture atlas, and then GPU colored/blit from there on.
It is just that instead of XAML it uses HTML/CSS, JS/C/C++ instead of C# and works on all desktop platforms.
For the note: historically Sciter appeared before WPF and WinUI/UWP : https://sciter.com/sciter/sciter-vs-wpf/
By any means WPF and WinUI are not UIs provided by the OS. These are userland UI libraries. Same thing as Sciter.
Without WinRT there isn't WinUI, while WPF really only depends on DirectX 9.
[1]: https://jeremiahmorrill.wordpress.com/2011/02/14/a-critical-...