GTK is a widget toolkit that uses Xlib, which Wayland doesn't have, so that's why it must be ported to Wayland.
Wayland is a tiny display server. It's using EGL? If so (don't realy remember with all that info flying around) then GTK+ will probably render to a texture/pixmap which Wayland will use (compositing) for drawing windows.
If they want to use it for meego, it shall be C++/Qt (erk!), wouldn't it?
(I'm sad to see the C dependency swapped for a C++ dependency in meego)