Don't do this on prior releases of the operating systems. There are reports of severe crashes (for me, there were only dependency errors but you never know)!
Gimp version 2.8 is now available as sourcecode and via ppa. It comes with an often requested single-window mode and a few other features. If you have any earlier versions of Gimp installed, purge those first:
sudo apt-get purge gimp*
To install it, add the ppa to your sources, update the apt database and install it:
sudo add-apt-repository ppa:otto-kesselgulasch/gimp
sudo apt-get update
sudo apt-get install gimp
To enable single-window mode, open Gimp and select Windows > Single-Window Mode