According to the Wikipedia page for Ubuntu, Ubuntu ran GNOME only until version 10.10, which is when Unity was released.
However, according to Sysinfo, my computer has GNOME.
I have Ubuntu 15.04.
Screenshot your desktop and we can tell you what version at-a-glance you're running.
If you're running plain old standard Ubuntu 15.04, then you'll have some GNOME components, but you're using Unity.
If you're running Ubuntu GNOME 15.04, then you're using the GNOME environment. This is a specialized variant, though, which has GNOME as the primary desktop environment and not Unity. (It's a different installer ISO as well)
Unless you have manually changed it, you should be running Unity. The only Gnome update which i read that will be applied is on the Software Center, removing the Unity one
Just look at your desktop. If you see a bar in the left side with some icons and your "close" and "maximize" buttons are in the left, you're running Unity. By default, Ubuntu comes with Unity, but there are tons of other Desktop Environments.