Work is underway on adding GTK+ and GNOME support to Gwydion Dylan. If you would like to help, please join the mailing list.
We'd eventually like to support DUIM (the Dylan User Interface Manager) as a wrapper around GTK+ and GNOME. This probably involves a huge amount of work.
Note that OpenDylan already has a GTK+ backend for DUIM. While this code has been suffering from bit rot for the last few years—the main problem being that it doesn't support GTK+ 2.0—renovation work has been started.