is GTK 3.x a real cross-platform solution?
2018-12-18 Update:
GTK+ 3 on Windows has been supported through MSYS2 for years.
Old answer:
GTK for Windows is currently provided "as-is". What it means, basically, is that there are no GTK maintainer that use Windows, and those using Linux have enough work maintaining the Linux builds.
This means that any contribution for GTK 3 on Windows is welcome, some people are using it, but that's not ready for the masses because nobody steps up to do the work, which won't be done by magic overnight. So it's usable, but don't expect reactive maintenance.
As for the build, there's a french dude providing an (unofficial) GTK 3 installer for Windows.
No it isn't - at least when you want to be on the safe side for the future. The MacOSX and GTK ports are less then perfect and completely voluntarily which means by a program of this complexity and size they are not not active developed. And nobody cares that old features are implemented before new are added.
Almost 2 years since GTK3 released and there is still no official Windows binary distribution. And on one of the GNOME conferences there was an open discussion if GTK4 should be made Linux only. Well i guess they mean Linux/BSD - but Wayland is pure Linux at the moment and BSD do have a lack of developer for Desktop stuff.
This should be enough to scare any person with a serious program which requires real world investment away.
Also i strongly belive that cross platform GUI toolkits are a thing from the past. Abstract the GUI of your app and develop for every platform. With the success of the AppStores you will be forced more and more to use the native platform style. Your app will already be rejected by Apples AppStore if it looks to different. Windows is now enforcing WinRT. Be prepared that this will just be more important in the future.
So the way to go is WxWidgets which uses native widget sets. Skip GTK
As of right now, no.
Like Frédéric Hamidi said, see Where can I download precompiled GTK+ 3 binaries or windows installer? for more info.