What some of the popes have said about Gregorian Chant:

Pope St Pius X: “(Gregorian Chant) is the only chant the Church has inherited from the ancient fathers which she has jealously guarded for centuries... and which she directly proposes to the faithful as her own.”
Tra le sollecitudini 1903.

Pope Pius XII: “Gregorian Chant is the... proper and principal music of the Roman (Catholic) Church... it is preferable to use it as against other kinds of sacred music.”
On Sacred Music and the Liturgy 1958.

Vatican II: “The Church acknowledges Gregorian Chant as specially suited to the Roman liturgy, therefore, other things being equal, it should be given pride of place in liturgical services”
Sacrosanctum Concilium 116. 1963.

Pope Benedict XVI: “It is possible to modernise holy music, but it should not happen outside the traditional path of Gregorian chants or sacred polyphonic choral music."
At a Sacred Concert 2006.