Since the latest version of GIMP has not been included in the main 12.04 repositories because it was not ready at release time you have to use a PPA at current.
sudo add-apt-repository ppa:otto-kesselgulasch/gimp
sudo apt-get update
sudo apt-get install gimp
TIP: Go to the “Windows” menu and select ‘Single-Window Mode’ to use Gimp in one window (see screenshot).