Richard Stallman intended GNU to be a full-fledged operating system, but without a kernel, he couldn't do it. At least until he discovered Linus Torvalds' Linux kernel, thus the GNU/Linux (or GNU+Linux) operating system was born.

As far as open-source projects are involved, both are open-source projects. And you don't have to only use the Linux kernel with GNU, as there's the GNU/Hurd project, and then the ever-popular Android (runs on a modified Linux kernel, the remaining are developed by Google)

First of all, it's GNU/Linux not just Linux(which is the name of the kernel). Second, most versions of GNU/Linux are free. Lastly, the most popular version is Ubuntu.

The accronym "GNU" stands for Gnu Is not Unix.

Technically speaking, it's "GNU/Linux" because Linux is the kernel, not the whole thing, we just shorten it to Linux (I do this myself, but I do know that Linux is just the kernel).

It is made up of the Linux kernel and several GNU programs (try typing a basic command into a terminal with --help, chances are you could easily find one that says GNU somewhere at the bottom).

GNU/Linux(same thing)

There's:

Red hat, Ubuntu, LFS(Linux from scratch) and many others

GNU/Linux? Plenty. Debian, Ubuntu, Arch Linux, Gentoo, Slackware, Fedora, and many more.

Non-GNU Linux? Android.