The release date situation is indeed difficult - I was first planning to use the date the game was made available on the GOG website. However, as far as I can tell, GOG.com doesn't provide that information (example for the Might and Magic 6-Pack:
http://www.gog.com/en/gamecard/might...imited_edition) If somebody finds that information, I'd be happy to amend my submissions.
Good point about GOG being the publisher, I'll change that for my previous submissions.