Weren't you downloading the demo in the fist place because you didn't have all the files you needed from the gog version? And I believe you were right, using the GOG files aren't enough. Perhaps just using the files and folders from the demo is enough, if you can't find all the files and folders listed there you need stuff from GOG as well.
But now that you manages to get the files from the demo, why don't you simply follow spartahawks steps
here. He made that topic specifically for people with GOG to get the official editor to run.
EDIT:
OK, good. What was it?