Taken from the CompTIA IT Fundamentals Exam Guide book (2nd edition, published 2021). I'm not sure if they fixed this in newer versions, if at all.
From Wikipedia, the free encyclopedia
Linux is a family of open source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991 by Linus Torvalds. Linux is typically packaged in a Linux distribution (or distro for short).
Distributions include the Linux kernel and supporting system software and libraries, many of which are provided by the GNU Project. Many Linux distributions use the word "Linux" in their name, but the Free Software Foundation uses the name GNU/Linux to emphasize the importance of GNU software, causing some controversy.
Community icon by Alpár-Etele Méder, licensed under CC BY 3.0
GNU is a project whose goal is to create a free operating system.
Way back in the dark ages when commercial UNIX was relevant, it was common to install various GNU utilities to get a better user experience or get things like a C compiler without having to pay tons of money for it.
The kernel part of the project didn't work out, so the de facto purpose of the GNU project morphed into creating better utilities and libraries for other operating systems.
When the Linux kernel came around, the GNU software was the base of system. That's why RMS insists on the GNU/Linux thing.
Things like the window system and desktop environment aren't really considered part of the OS by folks like RMS (and me, for that matter). It's probably an age thing - used to be there was a "core" system and various add-ons. The core system is the OS in our world view.