Robert Ancell has blogged about the work he and Ryan Lortie have been working on recently for allowing GTK+ applications to run in Unity 8 with Mir. Using their out-of-tree patches, some GNOME/GTK+ applications are starting to work natively, but there's still many items to address like GTK+ full-screen applications, cursor changes, and much more.
It's still not clear if upstream GNOME maintainers will allow the GTK+ Mir back-end to be added, but for now anyway the support is being developed through an Ubuntu Personal Package archive. Those wishing to try out the experimental support can do so if you're running the development copy of Ubuntu 14.10. More details via Ancell's blog post and his demo video embedded below.