MinGW is a Windows C/C++ compiler and toolchain based on gcc and binutils which runs on Linux, but generates Windows libraries (*.dll) and executables (*.exe). This means that when a customer asks you to write a Windows program, you can do it without leaving the comfort of your familiar Linux environment.
Today the Fedora MinGW Special Interest Group made a milestone release of the compiler toolchain and many basic libraries including Gtk and all its dependencies.
These aren't part of Fedora proper yet, but you can grab source RPMs and x86-64 binaries from my website here:
(Still uploading as I write this)
If you prefer you can clone our Mercurial development repository. (Read the README file)
The current draft packaging guidelines may look complicated but if you start from our example spec file (mingw-example.spec) then that takes care of most packaging scenarios and dependency generation automatically.