While there's already the Wayland back-end within GTK3, it isn't 100% feature complete and ready to be used by all. Among the missing functionality has been support for handling client side decorations, as used by the Weston compositor, rather than server side decorations as is common now for the Linux desktop.
Rob Bradford, Kristian Høgsberg, and other Intel Open-Source Technology Center developers have been working to bring "CSD" support to the tool-kit used by GNOME and others. Within the csd-for-review branch of the official GNOME GTK+ Git repository is their latest work, with the last commits being from last week.
This work adds in support for client-side decorations under Wayland. The support also allows using client side decorations on non-Wayland platforms by setting the GTK_CSD environment variable.
While GNOME/GTK 3.8 is due out before month's end, hopefully these Wayland improvements will be merged (along with other work) well in time for GNOME 3.10 later in the year. We're also looking forward to the revived GNOME Shell port for Wayland.