Why say "GNU+Linux" rather than 'Linux'?
One reason is that is most of the discussion here is not merely about the kernel used in software, it is about the Userland - all of those GNU applications used on a daily basis, and the distributions they form when they are packaged together.
Using "GNU+Linux" instead of just 'Linux' reminds the speaker and the listener of the spirit of freedom that led the development of our beloved operating systems, like Debian GNU/Linux.
When the opportunity arises, please do not neglect to remember freedom. Fly the flag, and be proud to use GNU!
(In the interest of freedom, it ought to be mentioned that 'Linux', the kernel, has not been free for some long while now. It contains binary blobs, unreadable, non-free, proprietary code, that was included for the sake of 'convenience'. Please remember to use Linux-Libre, where appropriate. Linux Libre is entirely free software. It is the Linux kernel stripped of blobs.)