I suggest waiting for us to add an official configuration for the -600, which we simply didn't had time to make it for the release.
PMDG planes requires some extra care, because of their custom variables so, even if you could edit them, you would still have to find all their custom variables and even if you managed to do that, you would still need to handle the custom parking brake and GSX editor doesn't have any interface to let you do that, even if it can, internally.
After we'll release a base standard configuration, you should be able to add your own edits, because the .INI adds on top of what's already in the GSX internal database.