Gimp installation can be done in the Ubuntu Software Center or Ubuntu Tweak, search just gimp .. then you know what's next
Well when gimp opened the window layout definitely separated, confusing isn't ? -_-
Follow this step if you want your gimp have in one windows, its simple just
sudo wget http://gimpbox.googlecode.com/hg/gimpbox.py -O /usr/local/bin/gimpboxThen you change the gimp to gimpbox by following this step :
sudo chmod +x /usr/local/bin/gimpbox
right click on menu bar - edit menu - click graphic - double click gimp - browse to /usr/local/bin/gimpbox
like this :
Finally done :D , enjoy it !