Peter,
Try adding back-ports to your /etc/apt/sources.list:
and then run apt-get update and then try updating gimp and see if it updates it.
I did not check specifically for gimp, but if there is no newer version of gimp in backports, you can try adding the jessie sources, however, if you do this you would be wise to restrict it to just gimp. Some other kind person here may be able to help you do this but it's not always easy/wise to do this in my experience since you can end up installing a ton of unexpected libraries and other tools from jessie into your weezy system. Consider updating to or installing a wheezy system if gimp there is the later version.
Michael Grant