I am wondering why Ubuntu is being called distro of "GNU/Linux" even though it offers proprietary graphic drivers (and some other things) which are not part of GNU GPL license.
From the "What is GNU/Linux?" section of the Ubuntu Installation Guide, which is copied from the Debian GNU/Linux Installation Guide (with my added emphasis):
The most important part of an operating system is the kernel. In a GNU/Linux system, Linux is the kernel component. The rest of the system consists of other programs, many of which were written by or for the GNU Project. Because the Linux kernel alone does not form a working operating system, we prefer to use the term “GNU/Linux” to refer to systems that many people casually refer to as “Linux”.
The key word in the section above is "many", which does not mean "all". A "GNU/Linux" system may contain non-GNU components.
For users that want to be made aware of any non-GNU software on their Debian or Ubuntu system, there is the
vrms package ("Virtual Richard M. Stallman"; untested by me personally though).
vrmsdoesn't list only non-GNU packages: it lists what Debian lists as non-free (which incidentally lists several packages using GNU sources such as the documentation). Jun 3, 2021 at 19:12
GNU refers to the programs that are in the GNU suite which most distributions, such as Ubuntu, include. For example, Ubuntu ships
coreutils which is a GNU suite.
Having proprietary parts does not exclude the distribution from including GNU pieces.
From GNU/Linux FAQ written by Richard Stallman
Why do you call the system we use GNU/Linux and not Linux?
Most operating system distributions based on Linux as kernel are basically modified versions of the GNU operating system. We began developing GNU in 1984, years before Linus Torvalds started to write his kernel. Our goal was to develop a complete free operating system. Of course, we did not develop all the parts ourselves—but we led the way. We developed most of the central components, forming the largest single contribution to the whole system. The basic vision was ours too.
In fairness, we ought to get at least equal mention.
See Linux and the GNU System and GNU Users Who Have Never Heard of GNU for more explanation, and The GNU Project for the history.