Gaa was ordained a priest for the
Archdiocese of Manila on September 23, 2000. His first assignments were as Parochial Vicar at Saint John the Baptist Parish, Immaculate Conception Parish, Presentation of the Child Jesus Parish, and Saint Anthony of Padua Parish. Between 2002 and 2003, he served as Parish Priest of Holy Spirit Parish in
Caloocan City. After returning from his studies in Rome, he became spiritual director and dean of studies at Holy Apostles Senior Seminary in Makati City in 2003. He was later appointed rector of the seminary in 2008 and became a member of the Presbyteral Council of the Archdiocese of Manila. == Episcopal ministry ==