I would assume that a non-GPL installer for GPL software would be fine, because it doesn't actually run the software, just installs it as a set of files (If it does run the program, generally it doesn't interact with it). I would equate it to using something like GNU 'indent' on proprietary software - I doubt using a GPL program to modify data would cause that data to have to be under the GPL or any other license.

What is very possible is that if they integrated their installer into GIMP's installer (Since GIMP already has it's own installer), GIMP's installer is GPL so their modification would be a GPL violation unless they make the code available. If all their installer does is run GIMP's installer though, then there's no violation AFAIK.