paraphrased Wikipedia quote:
In 1991, the Linux kernel appeared. Combined with the operating system utilities developed by the GNU project, it allowed for the first operating system that was free software, commonly known as Linux.
in other words, the linux kernel and GNU utilities work together to form the linux operating system. this is why people like to say “GNU/Linux” or “GNU+Linux”, to recognise the work made by the GNU projects.
however, it is possible to make a linux-based operating system based on the linux kernel. notable examples of this include alpine and android (yes, the phone operating system). this means that when you say “GNU/Linux”, you aren’t actually addressing all linux-based operating systems. android is not GNU/linux because it does not contain GNU software, yet it’s still a linux-based operating system.
non-GNU linux operating systems are rare, but they do exist. personally, i prefer to say “linux” rather than “GNU/linux” to acknowledge the existence and (more importantly) the viability of projects such as alpine, but there are people who like to recognise GNU’s contributions to the mainstream linux operating system base. both are correct. the maintainers of debian call it a “GNU/Linux distribution”, while arch bills itself as a “linux distribution”.