As a professional, Gren was suspended from the Swedish national team. However, as the World Cup of 1958 was to be played on home soil and the Swedish team seemed too weak for the competition, the Swedish Football Association changed this rule, and Gren, having now ended his professional career and playing in the Swedish Division 2, was again eligible. Gren went on to be the oldest player of this World Cup, 37 years old, and played an important role in the silver medal-winning Swedish team. He played five matches during the World Cup and scored one goal, 2–1 against West Germany in the semi-final.