If anything, Gregorian chant is about text. Its roots lay in the liturgical texts being read out aloud. When the congregations started to grow, the texts were recited (sung on one tone). This added to the comprehensibility, but the monotonous singing was pretty boring as well. To solve this problem, the more creative readers introduced accents : on syllables, which would be somewhat stressed in normal speech as well, they sung a higher note. Or they lowered the final note, just as your intonation drops at the end of a sentence. This technique is still visible in the simple recitations such as Dóminus vobíscum. Et cum spíritu tuo.
¶The readers had soon developed a wide range of ornamentations. Important syllables got something 'extra', the end of the sentences - the cadence - got a descending line of notes, to take the melody 'home' to a resting point. These beautiful 'primitive' structures can still be found in the lectures (Epistle and Gospel), prefaces, the Pater Noster, the litanies and of course in the many antiphons of the Divine Office.
But the people asked for more, and the singers wanted to show their audience what they were really made of. Gradually, the simple tones developed into highly complex melodies. Some syllables got up to twenty notes, and were called melismas. Of course, this enriched repertoire was only available to trained voices.
Despite these developments and the many grace notes we may encounter when singing Gregorian chant, the very basis of it all remains the text. The main priority of any singer or choir should always be clarity and intelligibility. Words, sentences and constituents are part of a larger whole and have to be sung that way. This is a typical feature of Gregorian chant: in Renascence music, for instance, music as such was far more important than the text, which was only a cause for the music to be written.
¶This 'rule' obviously has some important consequences as to performance practice. Gregorian chant is inextricably linked to Latin. There are some Greek texts in the repertoire (Kyrie eleison of course, but also the Hagios o Theos from the Improperia on Good Friday) but in general, the official language is Latin. Church Latin, that is: the mediaeval variant of the language which differs slightly from the classical variant (mainly in the field of pronunciation). Because of the importance of the text, it is an absolute must for the singers to understand it.
Gregorian chant is performed a capella, without accompaniment (not even with an organ). This rule is not always observed, as many congregations do not have the necessary musical skills and background, but choirs should try to sing a capella. Another feature of Gregorian chant is the fact that is 'monodious' : only one note at the time. This also means that it cannot be sung in octaves, i.e. not in mixed choirs, although not everybody agrees on this. Both sexes have got the same musical potential, of course, but should use it separately. The only alternative is to have men and women alternate (in a psalm, for instance).
So, in a nutshell: the basis for any Gregorian antiphon lies in its text. Therefore, understanding the text and the links between sentences and constituents is elementary. Only then is it possible to make some sense of the music itself.