GNU is an operating system that implements POSIX (mostly). GNU can be built with different kernels. HURD is one, but there was also Debian GNU/kFreeBSD for some time. You can also run GNU on Windows with WSL (though WSL 2 runs GNU on Linux in a VM). Similarly, Linux can be used with different user-land software. Many Linux operating systems are not POSIX. For example, Android is a popular Linux operating system. There are also POSIX operating systems that use Linux and a user-lands other than GNU, such as Alpine. Because this two things can be used together, but can also be used with different components, it's useful to refer to that combination specifically in many contexts. Fedora, Debian, arch, and gentoo are all GNU/Linux operating systems. Alpine, Android, and dd-wrt are all Linux operating systems. Generally, when I'm talking about a feature of the kernel, I'll refer to Linux systems, because the feature will probably be present in all of them. But of I'm talking about Unix-like systems, I'll usually refer to GNU/Linux to differentiate it from other Linux systems. More on reddit.com

The gnu project maintains a lot of different software projects . Some of those are more or less "ordinary applications" you might use or not use...for example gnu octave or gimp . But some of the software projects are essential for a working system. The most essential project is probably glibc . Every unix system needs a libc...and on gnu/linux distros that one is from the gnu project. The libc on your system is what pretty much every program uses to do "basic" tasks like writing files, getting time, controlling processes and threads, dealing with standard input/output, pipes...The libc either provides that functionality or redirects the request to the kernel in a unified way. It's really the most basic library in your system used by almost everything. Also every system needs a shell...and the standard in linux is gnu bash , a shell needs a basic set of commands like ls, cp, mkdir...and the standard there is gnu coreutils . On the development level gnu provides a compiler for several programming langauges, provides autotools used by many projects to deal with compiling for different target systems, provides the default make implementation for the actual compile runs... Most of the tools gnu provides have alternatives...but there isn't one project that provides all the functionality gnu has, you usually have to take stuff from many different projects to get something close. You can't have a working system without gnu or one of the alternatives. So lots of the tools gnu provides are the defacto standard on linux systems. More on reddit.com

what about simply using it in a commercial application and making commercial gain with its use? That's totally fine. That's what everyone from Google to Valve to IBM do. The only thing (within reason) you can't do is not share the source code if you distribute a modified version (and as u/ameoba says, GPL v3 makes it clearer what 'distribute' means). The GPL puts no limits on using free software commercially, it's not mentioned at all. More on reddit.com

I counted 4 different scheme implementations More on reddit.com