Writing
Default: IQ-5.
This is the ability to write in a clear or entertaining manner. A successful roll means the work is readable and accurate.
This is mostly useful to earn a living or write for GURPS, but can sometimes help on adventures...or after them. The report of a spy, soldier, or private investigator is far more useful if it is well-written!
Modifiers: The time modifiers under Time Spent will often apply; -5 if you are writing about an unfamiliar subject; Language modifiers.