It is a well known bug in the current version of the game. The game developers show no interest in fixing it, saying they've moved onto other games and don't plan to do any more work on Omerta, and GoG doesn't seem to care. Basically, the developers and GoG are knowingly ripping people off.

The official statement from Kalypso, the developer of the game, when asked about a patch to fix the problem is this:

"Tropico 5 was released just a little over a month ago, Omerta is now one and a half year old. Unfortunately we can't provide patch support on all games years after release, the team is simply not big enough to accomodate for this. Active development on the Omerta project has stopped and moved to Tropico 5."

It looks like, at least according to Kalypso, we will never see a fix to this issue.

That's a real bummer.
I would imagine that a fix for this bug would not be as time- and/or ressource consuming.
It's a shame that they sell this game at this state and don't attemp to make it playable.
Ok, the game is over a year old. But is the gaming industry this fast developing that a one year old game is that old that nobody would play this game...? I don't think so.

That reply from Kalypso was on a report of the Georgia Avenue game breaking bug in 1.06.
A bug that should have been fixed in the 1.07 version, but hasn't in the GOG version. See also http://www.gog.com/forum/omerta_city_of_gangsters/georgia_avenue_still_broken_in_107/post6
An unofficial fix to that has already been posted, which makes one wonder what's taking Kalypso so long to properly fix this bug in the GOG version.
The ending credits bug appeared in the 1.07 version. Some people on Steam reported it too, so it might not be limited to the GOG version. It's fixed when you install the Japanese Incentive DLC.

Kalypso did mention there would be a 1.08 version.
Nevertheless the way Kalypso is dealing with this, including the lack of communication, is quite disappointing.

Even dishonest.

You don't need that Incentive DLC, this is my patch:
www.sendspace.pl/file/d3eab490c9314609b7bb3ba
Just put this to your game folder and should work, It works for me.

SENDSPACE stopped working, so there is no way to dl the file. Props for making a patch though.

Here you are
http://catshare.net/OUr2siYwbdkB44Ny/Dlc.zip