Hello,
I've successfully managed to build Geany on MSYS2, against both gtk2 and gtk3. I've started to describe the steps on the wiki: https://wiki.geany.org/howtos/win32/msys2, however the page is not complete yet (but complete enough to produce a running geany).
Short introduction, see the wiki page for more details:
Msys2 is a successor to msys which offers a unix-like environment on Windows combined with a pacman-based package manager. It's purpose is to simplify win32 compilations, and it's doing great at that for GTK+ stack and related projects. In fact, it's so good it should become the default method of compiling Geany on Windows.
See http://sourceforge.net/projects/msys2/ and http://msys2.github.io/
For me, as regular Arch Linux user, this is by far the most easiest method to compile Geany on Windows. And it has the *huge* advantage of providing up-to-date binaries of the GTK+ stack, in contrast to the obsolete gtk bundles. I heard the GTK guys plan on removing the bundles and instead rely solely on MSYS2 for all of their win32 efforts.
Best regards