In the Software Centre, Gimp 2.6 is still the available stable release. I've heard talk about Gimp 2.8 being almost ready (and Gimp 2.7 being unstable).
Will Ubuntu 12.04 ship with Gimp 2.8?
The GIMP won't be installed default on 12.04 because Canonical wants the ubuntu image to fit on a cd. And the software repositories will contain GIMP 2.6 because GIMP 2.8 isn't stable yet.
You can install GIMP via the instructions here