So I downloaded and and updated to 1.07(thanks by the way!). I was previously stuck on the Garden Pier level on the main campaign mode due to the bug, and today I was looking forward to actually progressing further. I load my saved game and it automatically said I've won, but then the game credits started rolling. Did I really just beat the game in such an anticlimactic manner? ._.

Yes, congratulations u won! now go celebrate with a bottle of wine on the house and leave this pile of a mess of a game that caused many untold grief in the past behind and never look back again.
I hope u did the above, if not, then did u try to restart a new game anew and see anything different. possibly ur old save file is bugged by the new patch. I believe you've still ways to go in terms of completing the game since you're only at the first mission of Act II.

Bleh. I guess I'll start again and see if it's any different.

Started a new game and got the credits screen after I completed the first mission. Yeah...

That's really wonky. You may need to uninstall and reinstall...

Looks to me u can try what the other guy did in this thread, in the end he just rage-quit so I don't know how it would work out for u.

U can also try to start a new game if u still encounter the same issue with ur save file. If that doesn't work then i'm out of ideas sorry :<

It looks like the new bug introduced with the new patch 1.07 :
http://www.gog.com/forum/omerta_city_of_gangsters/cannot_continue_from_main_avenue_start_of_the_game
The game finish now at the end of the first level with this bug, so, I guess it should be correlated.