If you just installed Ubuntu, and did nothing to change it, you will be running Gnome.
However, some people refer to all the *buntus as Ubuntu and you could conceivably have actually installed kubuntu while installing something that you had been told was Ubuntu. Or, you could have installed Gnome Ubuntu and used package management to install kde and delete Gnome (but I'd have thought that you would have noticed).
Have a look in synaptic - you probably have mostly Gnome and Gnome packages installed and not KDE ones. That'll be Gnome, then.