أوليمبو
Mussis joined Gimnasia La Plata in the youth years. He received his first team debut on 15 April 2012 away against Gimnasia de Jujuy in the Primera B Nacional, after replacing Nicolas Alejandro Cabrera in the 74th minute. Despite not having many matches, Mussis became an undisputed pillar of the team. He scored his first goal on 10 March 2013 against Club Olimpo in a 3-0 victory. He was a permanent part of the Gimnasia La Plata team, won promotion to the Primera División on 28 May 2013 at the Estadio Mario Alberto Kempes.