I have tasted both Qt 4.x/5.x and GTKmm 3.x and I really like GTKmm over Qt.
Now I have just discovered that GTKmm 3.x doesn't offer a Windows porting, they also do not offer a Mac porting, basically I can't find nothing but libraries/sources/binaries for Linux. Apparently the Windows support ( I'm not sure about the situation for the Mac OS stuff ) was dropped in the 2.x era and it's still not-existing today.
I have planned to use GTK 3.x for specific reasons, especially for some new features introduced by the 3.x branch, so using GTK 2.x is an option off the table.
Now I ask: there is something new for the Windows and/or Mac support for GTK 3.x ? There is something ? even experimental projects ?
As of right now, no.
Like Frédéric Hamidi said, see Where can I download precompiled GTK+ 3 binaries or windows installer? for more info.