Unpacker:
https://gist.github.com/yancharkin/8cd1d49b053cbd59927c75f8df3b86dc Usage: python unpack_gog_installer.py "<INSTALLER.exe>"
It's using wine and innounp (because, for now, it's the easiest way to get 'install_script.iss'), but can be modified for using innoextract.
Script has some problems, but mostly works.
But there is another problem: looks like something wrong with some installers. For example, in new 'Arcanum' installer, size of main exe = 674K, and in old = 1.9M, and, of course, game fails to launch (even when properly installed).