Gullibility is one of Fallout GURPS disadvantages. See: Fallout GURPS.
There's one born every minute, and you're it. A gullible person naturally believes everything he hears; he'll swallow even the most ridiculous fish-story, if it's told with any conviction. In order to not believe a lie - or an improbable truth, for that matter - a gullible character must make a roll against IQ, modified by the plausibility of the story. A lie well-told, or involving something the character has no familiarity with - "My father is the chief of police in this town, and he won't stand for this!" - calls for a -6 penalty to the IQ roll. A lie concerning a topic the gullible character is familiar with - "Didn't you know they bred ducks in your village, Torg?" - calls for a -3 to the roll; and even a totally outlandish tale - "Of course the Eskimos are descended from Spanish conquistadors; everyone knows that" - will be believed if the character fails a roll against unmodified IQ. Furthermore, a gullible character suffers a -3 penalty on any Merchant skill roll, or in any situation in which his credulity might be exploited. A gullible person can never learn the Detect Lies skill.”— Steve Jackson Games GURPS sources
With the abandonment of GURPS, Gullibility was removed from the game