I saw a previous post on this topic, which did not seem to resolve the question. To call Gimp from a command line, I would use
$ open -a /Applications/Gimp.app <image-file>
but Scribus doesn't seem to want to allow that. I can get around the problem by creating a shell script called opengimp like this:
exec open -a /Applications/Gimp.app "$@"
The script must be executable (chmod +x <path to/>opengimp). Specify the full path when you set the Gimp external executable in Scribus Preferences.