The opening cinematic of GK3 shows the character on board of a train, gaining consciusness, and disembarking at a french train station. Later in the game I learn there is a whole backstory about the case, which is also mentioned in the first few lines of every synopsis I saw on the internet (including IMDB and wikipedia). The backstory being some scottish prince hiring Gabe to protect his son from some vampires (???), but I never had the chance to learn anything of that, apart from reading it on the synopsis. Where is this backstory told/shown in-game? Not in the intro movie, and not in the manual either, not even via in-game dialogues (in one instance Gabe is filling in his pal about the case, and the dialogue jumps forward, cutting the backstory part altoghether, as if the player already knew it).

It's in a graphic novel/comic book that isn't included by GOG for whatever reason.

The backstory for GK3 was told in a short graphic novel that was originally included with the original print run of the game. You can download that comic here: http://www.sierrahelp.com/Games/GabrielKnight/GK3Help.html Click the "graphic novel" link in the Extras section.

I agree, the way the game jumps into the story without any explanation is very confusing. I wonder why GOG doesn't include it with the bonuses for the game.

Thanks for the link!
But what a terribly mediocre comic that is, both in design and text... I can't believe these shallow dialogues and unbelievable characterizations are written by JJ herself (not to mention the logic holes: for example am I supposed to believe that Gabe drove the whole way from Bavaria to Paris on the motorbike he then uses to follow the bad guys? That thing is only there for the sake of coolness). Not at all a worthy entrée to one of the deepest, best written adventures. That doesn't affect the game itself naturally.

thanks for the link! it's clear now, but I find a bit absurd that GOG didn't include this in the downloads, being free and all.

Licensing?