I pulled out my Magical Adventure Game Finder - when i entered "third person", "majestic music" and "Gabriel Knight atmosphere" all i got was: Gabriel Knight 1, 2 and 3
So, i'd say - There's no place like home, and no games like GK games. Still, there are some other pretty good games out there. And, if by "majestic music" you're after something which also has opera in it - there are none, especially not The Opera which is solely composed for the game. If you can settle down with classical pieces of music you can try Toonstruck. Outcast is also known for usage of Moscow symphony orchestra.