First make sure you have all the needed stuff:
Now run this command to test it:Code:sudo apt-get install xnest metacity gimp
You may like to change the "1024x690" to something that better fits your screen.Code:Xnest :1 -ac -name GIMP -geometry 1024x690 & metacity --display :1 & gimp --display :1
If it works, you may edit your GIMP menu shortcuts to reflect that command.
Here's a screenshot: