In all the hustle and bustle to find church music which is "relevant" to modern ears, the name of Francis Poulenc (1899-1963) is one of many that seems to be unjustly forgotten. His music is thoroughly 20th century, and yet it possesses a kind of mouvement which is in perfect harmony with Gregorian chant.
Have a listen to his Salve Regina.
And to the Gloria from the Messe en Sol Majeur.
Poulenc converted to Catholicism later in life; I'm not sure of the exact time. That was when he wrote the penitential motets. The above Gloria comes from a time before his conversion. [Correction: Not true. His conversion took place in 1936; the Mass in G Major was written in 1937.]