The new version of GIMP didn't make it into the current Ubuntu LTS release, so here are some lines to install it (from an inofficial repo) easily:
sudo apt-get remove gimp sudo apt-get autoremove sudo add-apt-repository ppa:otto-kesselgulasch/gimp sudo apt-get update sudo apt-get install gimp
Then your installed GIMP can do YMCK, one-window-mode and much more :-)