While the graphics will look a bit rough on modern machines, the characters and story more than compensate in the opening game in a trilogy by Jane Jensen. The story follows the titular character as he seeks to uncover the truth about a series of murders and his own family history. The story is set in New Orleans, where Knight uncovers a voodoo cult. This is a terrific old school adventure. I still can hear Tim Curry's lazy Southern drawl in my head as he speaks of himself; he leads a cast of excellent voice actors.
This gem was followed up by The Beast Within, the only game in the series to feature FMV, which was all the rage at the time, and I think one of the few games to make it work really well. If I recall correctly GK 2 came on a whopping 6 CDs and had an equally compelling story that followed Gabe and Grace's efforts to investigate werewolves in France.
The third title, Blood of the Sacred, Blood of the Damned, was less well-received than the previous two (I think the heyday of adventure games may have passed--at least for that period in time), but still told a compelling story that will be very familiar to readers of Dan Brown.
I hope Gog gets them all.