As a player, Gallego debuted in the Argentine league in 1974 as a midfielder for Newell's Old Boys. After a rookie season in which he saw little action, he quickly made a name for himself and was given a starting spot and regular playing time during the 1975 season. Gallego played for Argentina at the 1975 Pan American Games. After being part of the Argentine national team that won the 1978 FIFA World Cup, Gallego stayed on at Newell's for three more years before being transferred to one of the two biggest teams in his country: River Plate.