I'm aware of four
GURPS 4th edition adventures, and one has been mentioned:
- Then Silence is a modern-day horror adventure set in England and intended for a mature audience (48 pages pdf).
- Heroic Age Adventures contains three linked adventures for GURPS Greece (33 pages pdf).
Also, I collected some of the best horror adventures, two are generic and five are for
GURPS 3rd edition:
Best Free Horror (non-Cthulhu) Adventures.