"GNU is the operating system" only makes sense when speaking about GNU Hurd or Linux-libre. The kernel is part of the OS, so in the case of ubuntu GNU can't be the operating system. But if Linux is the kernel, and GNU is part of the operating system because the OS is more than the kernel, then why is the OS GNU/Linux and not Systemd/GNU/Linux? And on Ubuntu you install most stuff with apt instead of using gcc, and use most software with X.org instead of bash, so the OS is really apt/X.org/systemd/GNU/Linux. Of course that name quickly becomes silly, it's much more practical to just call the OS Ubuntu (as that expresses a specific package selection that includes GNU, linux, and a large number of equally indispensable parts, as well as their update cadence and methodology)

A GNU/Linux distro can be referred to as an operating system. Not sure if you are memeing or not, but in conversation going into specifics about the GNU/Linux, or GNU plus Linux as you might like to call it, distinction, is not relevant here.

And in fact, I would say that saying that “GNU is the operating system” is even incorrect. GNU plus Linux is the operating system. Neither part by itself is a complete operating system.

So, the distro is the operating system.

Ubuntu is the operating system.