Many choirs will be looking for easier chant than is found in the Graduale Romanum and the Graduale Triplex, especially at first. A good place to start is with any of the major congregational hymnals. The Latin chants found there are intended for congregations, but it is likely that congregations are not (yet) able to sing them. The choir might sing easier Latin antiphons, Latin chant hymns, or chant hymns in English. Hymns are an easy place to start because the same melody is repeated for each stanza of text. Because the melody of a strophic hymn is formulaic and not intrinsically tied to the Latin text, hymns are the one part of the Latin chant repertoire that can be sung in any language.
Other easier collections for choir are the following:
Graduale Simplex This “simple gradual” was called for by the Second Vatican Council (SC, no. 117), and was issued in expanded form in 1975 (G-5393). Although it was intended for congregations, since the Graduale Romanum is too difficult for congregational use, the scope of this collection (and the fact that it is entirely in Latin) makes it rather unusable for most congregations. Its most practical use is as a choir resource, a fallback when the choir is not able to learn the propers from the Graduale Romanum.
As a gradual, this is a book for the Mass. For each Mass it contains five chants—the entrance, gradual psalm, Gospel acclamation, offertory, and communion. Each chant is in simplified form, with a short antiphon and several psalm verses pointed to sing with a Gregorian psalm tone. Rather than providing an entirely new set of propers useable on only one particular Sunday, this book provides “commons” useable throughout a season. For Advent, for example, there are two Masses, either of which can be used throughout Advent.
The editors did something highly innovative to find these easy antiphons. They raided the repertoire of short, easy psalm antiphons from the Liturgy of the Hours and found suitable texts. Oftentimes the texts of the antiphons in the Graduale Simplex are identical or similar to the texts of the propers in the Graduale Romanum. The melodies of the Graduale Simplex come from the Antiphonale Romanum, the 1912 Roman Office book.
Cantus Selecti, “selected chants” published in 1989 (G-4264) as an excerpt from the 1949 version. At nearly 300 pages, it provides a broad selection of chants for every season and for occasions such as Benediction of the Blessed Sacrament or devotions to the Blessed Virgin Mary. It does not provide Mass propers, Mass ordinaries, or the order of Mass, but rather a wide selection of hymns, sequences, tropes, and antiphons. A choir able to sing a repertoire this extensive would perhaps be ready to make use instead of the Graduale Romanum or Graduale Triplex.