The Credibility gap is a term that was widely used politics, journalism and public dialog in the U.S during the year 1960-1970. During that time, it was most often used to define public disbelief about the Johnson government's speeches and rules on the Vietnam War.
The Tet Offensive widen the credibility gap by emphasizing the credibility gap between what the American public was being stated about the war, and what images they saw on their televisions.