As a first check, could you tell us if typing rawtherapee on the terminal launches RT from the AppImage? This will confirm if the paths and symlinks are set up properly.
Next thing will be to run gimp from the command line, store the terminal output into a log file, and post the log here so that we can inspect it:
gimp >& ~/gimp-log.txt
You can use a file hosting website like filebin.net to share the gimp-log.txt file.