Gallegos started playing in Danubio F.C. youth system alongside Uruguay U15 and Uruguay U17. After his good performances with the Uruguay national football team youth levels, many well known European clubs showed interest in Gallegos. But finally, on 20 July 2009, he signed a contract with Atlético Madrid B. On 31 July 2011, he was loaned out one season to CF Badalona. After playing eight games in the 2011–12 season, Gallegos returned to his country. On 23 August 2012, he joined Peñarol on a two-year contract. On 9 September 2013, he was signed on a free transfer by Como Calcio 1907, where he played 10 games and scored 2 goals in the 2013–14 Serie C2 season. The following year, he changed teams again and joined Petrolul Ploiești in Romania.