1. Right-click the gimp.app and choose “Show Package Content”.
  2. Edit the file info.plist
  3. Just above:

Place this:

  1. Now in order for the OSX to update the change, make a copy of the gimp.app, remove the old and rename.
  2. GIMP fonts would not be in retina resolution. However the icons would still be blurry and need a different fix.

Source: jayway.com