I didn't thought about the details much yet. Yes, the GTK bundles used by the script are a bit outdated, 2.24.10 is of 2012.
So maybe it's worth trying to get the MSYS2 packages for the GTK stack and use them. I guess the Mingw compiler toolchain can be used from distro packages.
After a quick search, I just found https://github.com/fracting/msys2-scripts which seems to run MSYS2 within Wine on Linux. Not really sure if this is a good idea, especially in terms of performance. But this way we can re-use most of the Windows build related scripts.
Or we try to fetch packages from the MSYS2 mirrors, extract them and setup paths accordingly. This is probably a cleaner way :).
I have a bit more spare time in October and hopefully can work on this then.