For Windows there are installers available for the Gtk+ runtime:
For installing Gtkd on windows see the Installing on Windows page in the wiki.
For testing and unstable, a recent version is in the official repo.
apt-get install libgtkd-3-dev
Debian, Ubuntu, LinuxMint, etc:
GtkD binaries are available in the APT repository for D
Since Fedora 17 GtkD is in the official repo,
all you have to do is:
yum install gtkd