GNU gives you the basic tool. Things like cp, mv, mount, gcc, GRUB. Those sorts of things. The tools that are small and do a single job are generally GNU and common across any *nix box you sit at.
Other user space stuff like Xfree, your email client, KDE are made by different groups.
The distro maker packages all these together to make it user friendly. They give you GUI config tools, a nice installer, themes, support, package management tools etc etc
They often modify the Linux kernel itself, doing things like 2.5 backports.
All distros are essentiall GNU/Linux it just depend who you are talking to as to whether they call it that.
The hardcore GNU people still regard Linux as a stop-gap until the HURD is fully ready, although the nay-sayers whinge that HURD is just vapourware.
Maybe you should check out
www.gnu.org