Gtk+3 for Windows

Gtk+ is a framework for building platform-independent graphical user interfaces. This is a setup program for installing Gtk+ Version 3.6.2 and all of its dependencies on Windows.

Download Gtk+3/gtkmm for Windows
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13 MB, Windows XP or higher, 32bit or 64bit
MD5: 6d1fa8dd37a2d04b6aedb9f18b909417

The installer must be run as Administrator.
The installer sets up everything required for running Gtk+ and gtkmm based applications. See here for more details.
See here for the developer version of the package (includes headers etc.).
See here about how to create your own Gtk+/gtkmm applications.