Truthfulness is one of Fallout GURPS disadvantages. See: Fallout GURPS.
You hate to tell a lie - or you are just very bad at it. To keep silent about an uncomfortable truth (lying by omission), you must make your Will roll. To actually tell a falsehood, you must make your Will roll at a -5 penalty! A failed roll means you blurt out the truth, or stumble so much that your lie is obvious”— Steve Jackson Games GURPS sources
With the abandonment of GURPS, Truthfulness was removed from the game