Flutter is the easiest way to write GUI applications on Fuchsia today, but by no means the only one. There already exists a Wayland to Scenic translation layer. Adding support to Gnome or KDE is totally possible. There is even a rust GUI framework under development in the fuchsia codebase called Carnelian. Like Linux, there is no concept of native platform widgets on Fuchsia. It is up to a product developer to build up those types of layers. The Workstation product may build up such a toolkit of native widgets, but that won't bound the possibilities of what you can do with Fuchsia.