Which tutorial? The builds that were once hosted on win32builder.gnome.org were native GTK3 builds for Windows, so they would not help you for cross-compilation. Additionally, we stopped providing native builds for GTK3 years ago.
You need to install the appropriate MINGW cross-compilationpackages for your Linux distribution. Fedora calls them mingw64-gtk3, while Debian calls them mingw-w64. For other distributions you will have to ask the appropriate distribution’s support channel.