I've posted a pre-release of GIMP Portable 2.2.15. I'm doing a pre-release as it's now using a PortableApps.com Installer and combines the EN-US and multilingual packages into one installer. It will install over an existing English or Multilingual install. If you install over a Multilingual without the translations, it'll even clean up the installed ones... saving you space.
It's an 11.6MB download and will be 18 to 36MB installed depending on whether you include the translations. Please give it a try and post any feedback.
Download GIMP Portable 2.2.15 Pre-Release
(may take a few minutes to hit the download servers)