Gabriel Knight is the central protagonist of the series. Aside from his double duties as a novelist and supernatural investigator, Gabriel is also the proprietor of St. George's Rare Books in his hometown of New Orleans. Charismatic, sardonic, and intuitive, he has a way with women that has at times proven to impede the progress of his cases. When the series starts, Gabriel is a down on his luck author whose instinctive drive to solve a killing spree ultimately leads him to discover his family's ancient heritage as a line of the Schattenjägeren (Shadow Hunters). Upon the conclusion of his first case, Gabriel moves to Bavaria to reluctantly take up his birthright. His next two cases as a Schattenjäger are chronicled in the second and third games of the series.
By GK3 Gabriel has taken on the job of a Schattenjäger full-time. He is hired by Prince James of Albany (in the GK3 graphic novel) to protect his infant son, Charlie, from a group of vampires called "The Night Visitors", later revealed to be the "Adepts of the Holy Blood". Charlie is kidnapped as Gabriel becomes paralyzed (through unknown means). He follows the kidnappers to a train where he is knocked out, waking up at the Couiza station and making his way to Rennes-le-Château where he begins to investigate the area and tour-group staying at the Rennes Hotel.
Eventually, it is revealed that Prince James is a blood-descendant of Jesus and Gabriel confronts the Adepts in the rebuilt, underground Temple of Solomon. In the aftermath of his battle with the demon Asmodeus and the death of Montreaux (the head-vampire), Gabriel, Mosely and Mesmi discover the body of Jesus that is then carried off by Emilio Baza through a shining portal. When Gabriel returns to the hotel he discovers that Grace has left for India. It is also revealed that Gabriel is a descendant of a Roman soldier who nailed Jesus to the cross, and who accepted the duties as Schattenjäger for himself and his descendants as penance. The Schattenjäger talisman and dagger being made by blessed steel turned to gold after having been touched to Jesus' lips while he was on the cross.
Source: Wikipedia, "Gabriel Knight", available under the CC-BY-SA License.