GNU HURD, as you wrote, is the name of one of GNU kernels.
I think you meant GNU/HURD, an instance of the GNU operating system running on top of the HURD kernel.
GNU/Linux is another instance of the GNU operating system, using Linux as the kernel
ditto GNU/Linux-libre, GNU/Darwin, GNU/NTKERNEL, ...

there are also GNU/kOpenSolaris, GNU/kFreeBSD, ... that use kernels that don't have their own names.

they're all essentially the same operating system. there are differences brought about by the kernel running underneath, but for most users, depending on the quality of the ports, they are hardly noticeable

far more visible are the differences when you replace GNU with a different operating system, such as on Android/Linux, Windows/NTKERNEL, or MacOS/Darwin.