Hello vonH. First download and install msys2.
[https://msys2.github.io](https://msys2.github.io) Then open the msys shell and execute pacman -Ss gtk3 in response to get the list of packages mingw-w64-i686-gtk3 - for x32 mingw-w64-x86_64-gtk3 - for x64 pacman -S mingw-w64-x86_64-gtk3 In the latest version of msys2 something is broken and the packages are not installed correctly. Pacman downloads packages to C:\msys64\var\cache\pacman\pkg and then outputs "error: failed to commit transaction". I can only offer you a dirty hack pacman -S mingw-w64-x86_64-gtk3 --force Don't forget to add C:\msys64\mingw64\bin (and \lib) in the path variable.
