Porting XFCE applications (Thunar etc.) to GTK3 indeed gives automatic (or near-automatic) Wayland support. This means that XFCE applications will run natively (without XWayland) in Wayland compositors (such as sway, GNOME or KDE).

However, xfwm (the window manager) will also need to be ported to Wayland — i.e. become a Wayland compositor — which will probably be a more complicated undertaking, as "xfwm-wayland will need to fulfil the role of both X and the window manager. (Yes, using libraries like wlroots or libmir, will make this not completely impossible, but it will still be non-trivial.) Plausibly, XFCE might join forces and use the same Wayland compositor as, say, LXQt or Mate.

The Xfce port to GTK 3 is almost complete. Xfce 4.14 should be released sometime in August this year if all goes well.

https://wiki.xfce.org/releng/4.14/roadmap#roadmapplanned_fea...