Compiler and toolchain, and all the 'standard' UNIX tools: the shell, the text utils like cat, grep, awk, etc.
Basically, back in the 80s, the FSF, reimplemented what was at that time nearly the entirety of what was called UNIX except the kernel (which was what the HURD project was/is). It was to be the GNU OS. While the kernel was in development, the userspace tools were developed and ported to other UNIX systems like sunos as a replacement for the often deficient historical versions supported by the UNIX vendors.
So when Linus came along and wrote a UNIX-like kernel using gcc, he could load all those programs on and have a mostly functioning UNIX environment. This was the reason RMS objected to calling it just Linux, at that time the majority of the code running on the system was GNU. It was probably a legitimate point at the time. And even if there were a different compiler, without a set of userspace tools that people could freely get and use it is unlikely Linux would have been able to take off.
Now, of course, a huge part of the user experience is provided by X11, the desktop environments, and various graphical appliations. GNOME is part of the GNU project but X.org, KDE, and most of the applications are not. So it isn't really true that GNU software is still the majority of the OS. Of course, the kernel is even less important in terms of the user environment, and despite all the other software around it, GNU utilities are what makes it (not) UNIX.