Barefoot_Monkey: Anno 1404 and Dawn of Discovery are functionally the same game, but they are registered in Steam as two different products. Anno 1404 is steam application #33250 while Dawn of Discovery is application #33210. Connect is searching for #33250.
Either the publisher (Ubisoft) wants to promote one and not the other (which would be odd) or this is simply an oversight, and GOG needs to add a check for appid 33210 (possibly after checking with Ubisoft that it's OK to do so).
It might be worth sending a message to support to let them know that Dawn of Discovery isn't being detected as Anno 1404 by Connect, just in case nobody else has done so yet.
Tauto: Technically Dawn of Discovery and Anno Venice are two separate games.
If that is what the OP is trying to say or if I'm wrong then I apologize.
Venice is the expansion for 1404/ Dawn of discovery. In North American for some reason they decided to change the name.