I could write pages and pages about the Gabriel Knight games, but I'll just say they are must plays. Among many reasons what I love is how each game is different in style and execution.
GK1 - 2D, VGA; dark supernatural/gothic feel w/ Voodoo backdrop
GK2 - FMV w/ real-world backgrounds; more romantic and primal
GK3 - full 3D, time block structure; rich, complex conspiracy-centered story
GK1 is the game that made me truly love games. GK2 is one of the best games that used FMV w/ actors. GK3 is grossly under appreciated, partially because it's 3D doesn't look great and turns people off and partially because of that goddamn Old Man Murray article

. I always thought the 3D looked fine. Facial animations are pretty good. The environments are well detailed. Some character models are goofy (Mosely) but these things don't detract from the gameplay. I didn't think GK3 looked significantly different from most other 3D games at the time (1999).
GK3's interface is fantastic - independent camera movement that allows you to explore each area efficiently. You don't have to wait for the character to walk from point a to point b if the camera faces away from them - Gabriel or Grace will appear right behind the camera. The point-and-click interface is great, too, with various context-sensitive icons. It's possibly the best integration of 3D environments/exploration with classic point-and-click gameplay. Le Serpent Rouge is an amazing puzzle - challenging, multilayered, and hugely integral to advancing the story. Overall the puzzles and variety are very good, mustache aside (which sticks out but isn't THAT terrible).
I've long held GK3 is quite underrated. It's hard to live up to the expectations set by its predecessors but I think it manages to do that. Sometimes I think it's my favorite of the trilogy, but each game is special to me for different reasons.
Lastly, here is a really enjoyable series on the making of the Gabriel Knight games. Each article is an easy read - compact and informative. It's also a nice slice of the way adventure games or even PC gaming overall was changing in the 90s.
Hunting Shadows: The Making of Gabriel Knight
Chapter 1: Enter the Schattenjäger
Chapter 2: The Voodoo Queen
Chapter 3: Gaming Goes Hollywood
Chapter 4: Movie Magic
Chapter 5: Dinosaurs