I'd recommend to start with Scratches, even though it's 1st person. The game is well-written, well-designed, VERY dark, made with love and care. Sherlock Holmes vs Jack the Ripper is also a nice detective/thriller and the best ag by Frogwares for sure.
EDIT Oh, I didn't notice The Gabriel Knight series in your post! You should most definitely try them, all three games are brilliant examples of what you are looking for