> You would want to GTK (or Qt) I would think to draw the top level window and perhaps system menus, etc. even though the UI itself is drawn using a GPU canvas.

No, you would want to draw for Wayland or X. GTK and Qt themselves don't burden with importing each-other to work, for example.

My guess is that they import GTK only to get a title bar on GNOME, as GNOME forces applications to render their own. They could go custom and cut the dependency but it never looks quite right when apps do that.