is it all of the game or is it more that you need to buy???

All except the latest DLC (The Japanese incentive). Do note though that the GOG version has a game-breaking bug that Kalypso still has not provided a patch for (even though they patched the Steam version months ago). Kalypso says they're working on another patch to be released for the GOG and Steam versions, but that remains to be seen.

The game that was on offer for free (it's now expired) is Omerta base game and the first four DLCs. It does not include the latest DLC (The Japanese Incentive). So yes, if you want everything, you need to buy either the edition that was on offer (Omerta + 4 DLCs) as well as the Japanese Incentive or the Omerta Gold edition which has everything.

And yes you'll have to wait for the patch in order to be able to complete the game. Patch 1.06 broke the game. Patch 1.07 fixed it, but was only released for the Steam version. GOG is still waiting. Kalypso has said they're working on patch 1.08, which will be released for both Steam and GOG versions, but it has been several months and still no sign of it.

Hope that clears it up. :)

*edit* For clarification, the edition that was up for free was this one: Omerta City of Gangsters + 4 DLC. If you got that edition, then you need to grab The Japanese Incentive to have everything.

If you don't have either of those, then you can grab Omerta City of Gangsters Gold Edition and that will give you everything.

Again, either way you do it, the GOG version of the game is broken and needs a patch.

yes thank you soo much :)