Alpine Linux and many other distros use musl and busybox (non GNU projects, although busybox is GPL). And I would definitely call them Unix systems, since they

Alpine Linux and many other distros use musl and busybox (non GNU projects, although busybox is GPL). And I would definitely call them Unix systems, since they run almost all the same software. Although GCC is still a good argument, since very few distros use Clang by default, I think mostly because of the Linux kernel.