Angel Heart

Well, if you want to look at what sort of influences she had, try this one:

Sins of the Fathers involved magic.
The Beast Within involved werewolves.
Blood of the Sacred, Blood of the Damned involved vampires.
The fourth game, had it been made, was rumored to involve ghosts.

White Wolf Game Studio's first four RPG games were Vampire, Werewolf, Mage, and Wraith.

It's probably just coincidence, though (regardless of what White Wolf seemed to think when suing Sony over the Underworld movies) - the myths themselves have been around for ages, and it doesn't take THAT much effort to decide to use them. The key is using those ideas to write an effective story, which Jensen did in these games...

I just saw 'Angel Heart' the other night (great, great film, by the way), and I'd have to disagree with you about the similarity between the film and 'GK1'; with the exception of it being a mystery involving New Orleans Voodoo, it bared very little resemblance.

First off, while Mickey Rourke's Harry Angel character does look like Gabriel Knight, I find their personalities to be vastly different. Angel is a tough, hard-boiled detective who can be rather aggressive and threatening when he's trying to squeeze information from people, while Gabriel Knight is much more laid back and gets by on his charm and charisma. You can feel sympathy towards the character of Gabriel, but not so much Angel.
As far as Lisa Bonet's Epiphany Proudfoot character goes, she does LOOK identical to Malia Gedde, but differs in personality. Malia is a sultry, mysterious socialite and Epiphany is a much younger, seemingly more vulnerable girl living in poverty. She doesn't seem to reflect any sort of dark or ominous tones, at least not to the extent Malia does. Although the relationship between Angel and Proudfoot does remind me a bit of the relationship between Gabriel and Malia.
As far as the attractive assistant who helps Harry Angel out, nearly every old-fashioned P.I. has one of those. However, unlike Grace, the woman in 'Angel Heart' is featured only briefly and isn't that heavily involved with the story.

And, of course, the plots are completely different: In 'Sins of the Fathers', Gabriel investigates into a series of murders caused by Voodoo people of which Gabriel soon gets involved into, where as, in 'Angel Heart', Harry Angel is hired on a missing persons case which leads him to New Orleans and the case eventually involves a secret voodoo society. Of course, that's just the bare bones descriptions of the two, as both of them have deeper plots, but they're still totally different.

And, um...I guess that's all I have to say.