Historically, naming rights for an OS go to whoever actually puts together and distributes the complete system. For instance, if a workstation company licensed Unix from AT&T and ported it to their workstation, they got to name that OS whatever they wanted. A couple examples of this were Uniplus+, which was UniSoft's Unix, and 386/ix, which was Interactive System Corporation's Unix. Both were Unix systems--they used a Unix kernel and Unix utilities--but that wasn't their names. Half the fun working at a Unix workstation company in the early '80s was thinking of a neat name for your Unix port. :-)
For the complete systems distributed by Canonical, Red Hat, and the like, they are the ones who get to name the operating systems that they distribute. Ubuntu calls their OS the "Ubuntu operating system". Red Hat calls their OS "Red Hat Enterprise Linux".
Yes, they are also GNU systems, but if we want to be historically accurate, the most correct way to view this would be to view "GNU system" and "GNU/Linux" as specifications for a specific Unix-like userspace and for an OS that runs the GNU system on a Linux kernel, respectively. The Ubuntu operating system complies with the GNU system specification and is a GNU/Linux system, but it is named Ubuntu operating system.