Thanks to the great Stanford Renewal Database, we now know that every book with the word Gregorian in it published before 1964 is public domain, with the exception of these

RE299054

Gregorian chant.

Willi Apel.

A345013
Long record
R159517

Catechism of Gregorian chant

HÜGLE, GREGORY

A1018733
Long record
R315586

By Joseph Schrembs, Sister Alice Marie, Gregory Huegle, Mabel E. Bray & others

THE GREGORIAN CHANT MANUAL OF THE CATHOLIC MUSIC HOUR

A86300
Long record
RE467491

Square notes; a workbook in Gregorian chant.

Marie Margaret Rose, pseud. of M. Judith,

A557906
Long record
R402172

Accompaniments for chants in The Catholic music hour and The Gregorian chant manual

SCHAEFERS, PETER H Peter H. Schaefers, Joseph Schrembs, Sister Alice Marie & Gregory Huegle

AA298271
Long record
R362218

Examples of Gregorian chant and works by Orlandus Lassus and Giovanni Pierluigi Palestrina

SODERLUND, GUSTAVE FREDRIC, comp

AA249970
Long record
R292622

A short treatise on Gregorian accompaniment according to the principles of the monks of Solesmes

BRAGERS, ACHILLE P

A79397
Long record
R447251

Examples of Gregorian chants and works, by Orlandus Lassus, Giovanni Pierluigi Palestrina & Marc Antonio Ingegneri. Gustave Fredric Soderlund, compiler. (Eastman School of Music publication, no. 4) NM: revisions

LASSUS, ORLANDUS

AA367773
Long record