Following World War II, Gereben played in several Hungarian Chess Championships. In 1947, he won team gold medal and individual silver medal at sixth board in the 2nd Balkan Games in Sofia. In 1947, he took 7th in Vienna (2nd Schlechter Memorial; Szabó won). In 1948, he took 5th in Bad Gastein (Erik Lundin won). In 1951, he won in Sopot. In 1952, he took 15th in Budapest (Paul Keres won).
Due to the Hungarian uprising in 1956, Gereben emigrated to Switzerland and began playing extensively in the west. In 1957, he took 2nd, behind Gedeon Barcza, in San Benedetto del Tronto. In 1958/59, he took 3rd in Hastings. In 1959, he tied for 1st in Bognor Regis. In 1960, he tied for 4-5th in Zurich. In 1963/64, he tied for 1st-4th in Reggio Emilia. In 1967, he tied for 2nd-5th in Amsterdam (Master Tournament). In 1969, he took 4th in Monte Carlo (Master Tournament).
Gereben played four times in the Chess Olympiads; once for Hungary at first reserve board (+6 −3 =6) in the 11th Olympiad at Amsterdam 1954, and thrice for Switzerland; at third board (+5 −4 =7) at Siegen 1970, at second reserve (+3 −5 =5) at Skopje 1972, and at first reserve board (+3 –3 =7) at Nice 1974.
He remained a keen player in his adopted country even into his seventies. The British Chess Magazine editor, Bernard Cafferty, described him as "a delightfully friendly personality, rather different to many of the continental masters of that time. Chess was fun for him, not just a way to make a living."
Gereben was awarded the International Master (IM) title in 1950.[1] He died in Switzerland in the year of his 81st birthday.