Concerto Pastorale by Johann Melchior Molter, adapted for organ solo by Franz Lehrndorfer.
This article includes short chant-based organ works for Advent and Christmas, for Lent and Easter, for Pentecost and the Feast of Corpus Christi, as well as Hymns to Mary.
Here you will find "Short preludes and organ works based on chants" by Franz Lehrndorfer.
The article includes, among others, choral arrangements of chants for Lent.