He began performing at 13 and recorded with his jazz trio at 16. In the 1950s and 1960s Gaslini performed with his own quartet. He was the first Italian musician mentioned as a "new talent" in the Down Beat poll and the first Italian officially invited to a jazz festival in the United States of America (New Orleans 1976-1977).
He collaborated with leading American soloists, such as Anthony Braxton, Steve Lacy, Don Cherry, Roswell Rudd, Eddie Gómez, Max Roach, Nacci Alberto, but also with the Argentinian Gato Barbieri and Frenchman Jean-Luc Ponty.
He has also adapted the compositions of Albert Ayler and Sun Ra for solo piano, which the Soul Note label has issued. He also composed the soundtrack of Michelangelo Antonioni"s Louisiana notte (The Night, 1961).
From 1991 to 1995 Gaslini composed works for Carlo Actis Dato"s Italian Instabile Orchestra, and was the first holder of jazz courses at the Santa Cecilia Academy of Music in Rome (1972-1973). As to contemporary music, he composed symphonic works, operas and ballets represented at the Scala Theatre in Milan and other Italian theatres, in addition to film scores, including Your Hands on My Body (1970), Cross Current (1971), The Hassled Hooker (1972), The Night of the Devils (1972), So Sweet, So Dead (1972), When Women Were Called Virgins (1972), Five Women for the Killer (1974) and Kleinhoff Hotel (1977).
In 1975 he was contacted by Dario Argento to score Deep Red, having previously scored Argento"s The Five Days in 1973.
Unfortunately Argento was disappointed by the music, and only three of Gaslini"s original themes were retained.