there is no detailed instruction yet, but all the info needs to be added here:
http://www.gogwiki.com/wiki/Installer_Information

So assuming one wanted to add the Atlantis 2 and Atlantis 3 information, what would he have to do? Get the hash, and then check the installer properties for version? What about the date?

Also I just added the last missing Wiki page - that is, for the games that are no longer sold by GOG.

Yep. :p

Went ahead and added the values, the "modified date" I used are the ones from when I downloaded them, not sure if that's what we are supposed to use.