Can Intel Predecessor Content ("DLC containing extra missions from a promotional deal between Ubisoft and Intel") be obtained with GOG version? I haven't purchased the game yet. There seems to be even more exclusive content
This guide is for the Steam version: https://steamcommunity.com/sharedfiles/filedetails/?id=199722774

I'm pretty sure you can unlock this.

Haven't played in awhile, but I did 'unlock' via some fashion a 'predecessor tapes' quest (in addition to the included 'Jackel' tapes quest) - BUT it's bugged out (so is the Jackel tapes quest). You can only find so many of the tapes - then the 'mail' icon stops appearing and the quest stalls out at maybe 4 or 5 tapes found (somewhere in the 2nd-half of the game).

Been awhile so I don't recall exactly what instructions I followed - but that 'Intel Predecessor Content' sounds exactly like it. And it was just a matter of plugging in the appropriate unlock code.

Sadly, just don't expect to complete it; but it will give you a few extra things to do until the stalling-out, and it won't affect your ability to complete the game.