He was a French international rugby union player. His usual positions were, wing three quarter, centre three quarter, fullback and fly half. In fact he was so versatile that he played every back position for France with the exception of scrum half. He was a part of the France national rugby union team which won its first Grand Slam in the 1968 Five Nations tournament. Gachassin played as an international for France from 1961 to 1969. He was nicknamed Peter Pan and several other nicknames because of his diminutive size. He is one of the most exciting backs France ever produced and scored some of the most spectacular tries of the 1960s.