For a fair amount of time now there's been work on client side decorations for Wayland
so that the Weston compositor with GTK+ can do the window decorations on the client-side rather than server-side as done with the X.Org Server. That work has now been merged to master.
For being a feature to GTK+ 3.10 when released in September, client side decorations (CSD) are now ready for Wayland. The initial Wayland CSD work for GTK+ was done by Kristian Høgsberg and Rob Bradford at Intel. The initial commit can be seen here
A follow-on commit
added support for enabling GTK+ client-side decorations on non-Wayland platforms. This CSD support can be turned on for other platforms via the GTK_CSD