I think some parts of it are not GPL, outside the compiler. When I installed their combo distro on Windows, it said nothing about GPL, but presented a free-to-use kind of license.
Or maybe they’ve relicensed it as GPL but forgot to update the script that packages their Windows build. Not sure.
Strange thing is that it doesn’t put the license text file somewhere on the disk after installation. So to double-check, I’m going to have to uninstall and reinstall again.
The installer also has the dubious merit of shipping zero documentation. It’s all on-line.