CPDL provides the text and an English translation:
Gaudeamus omnes in Domino diem festum celebrantes
sub honore Sanctorum omnium:
de quorum solemnitate gaudent angeli, et collaudant Filium Dei.
Exsultate iusti in Domino: rectos decet collaudatio.
Let us all rejoice in the Lord celebrating the feast
in honour of all the saints,
in which solemnity the angels rejoice, while the Archangels praise the Son of God.
Ring out your joy to the lord, O you just; for praise is fitting for loyal hearts.
Glory be to the Father ...
(Although, you know: I do believe that the singers, "Collana Diretta da Bonafacio G Baroffia," have hit a wrong note there in that video; they forgot to sing the flat. Here's how the Benedictines of Clear Creek sing Gaudeamus for the Feast of St. Benedict:
And here's an mp3 from the Brazilian Benedictines for All Saints, which also contains the flat. Just to get things straightened out.)
Gaudeamus is used as the Introit for a number of saints' days during the Great Church Year (see note below); there are some variants included at that link, so you can see how the text is adjusted for other feasts. It's quite a beautiful text, especially this one, I think.
Here's the full chant score:
As I've noted before, the use of Gaudeamus as Introit for various saints' days apparently began with The Feast of St. Agatha in the 3rd Century. It sounds to me as if, after the chant had been used in that way for some time, it seemed natural to use it at All Saints as the Introit, too, again tying the Church Year together via the liturgy and its chant propers. Here's a quote discussing "Josquin's Mass for All Saints and the Book of Revelation" in a book titled Symbolic scores: Studies in the music of the Renaissance:
It should be pointed out, however, that the Introit "Gaudemus" - as ascertained already by Helmuth Ostoff - is also used for a large number of saints' days as well as for the feast of All Saints (November 1). The Introit sung in the Mass of Saint Agatha (February 5) is the oldest version. Its text is the same as that of Example 1, except that it has "Agathae martyres: de cujus passione" (Agatha martyr, at whose passion) instead of "Mariae virginis: de cujus assumptionae" (the Virgin Mary, for whose Assumption). From the eleventh century onwards, the antiphon appears in at least seven other Masses. In the Introit of All Saints the text passage quoted above reads "Sanctorum omnium de quorum solemnitate" (of all the Saints, at whose solemnity).
Below is the "Example 1" referred to above:
The videos below are recordings of that Josquin mass - called Missa Gaudeamus - referred to in the link above; wow, it's just completely gorgeous. It's based on this Introit, and the motif runs throughout.
This video has the Kyrie and Gloria:
This one the Credo:
This the Sanctus, Benedictus, and Agnus Dei:
No information about the musicians is given at the YouTube page, but I believe - from the CD cover art included - that it may be "De Labyrintho & Walter Testolin" - which would be this, I think.
There's some really interesting stuff at the link above, about this Mass specifically:
Although the Mass combines the techniques of cantus firmus and ostinato, it is generally the incipt of the Introit which, thematically, occupies the foreground. In the Ms. Cambrai 18 the motif even appears several times with the word "gaudeamus" instead of the litugical text. The ostinato technique has its culmination in the final Angus dei, where, as Jeremy Noble remarks, "the memorable opening phrase of the introit it put through a vertiginous series of transpositions." It has never been noticed that the arrangement of the total numbers of "Gaudeamus" statements in the five sections of the Mass is anything but proportional. The motif occurs 6, 14, 2, 5+7, and 4+23 times respectively. If we take into account that it appears more often in Agnus dei III, which is 58 bars long, than in the Kyrie and Gloria which together make up 220 bars, and that it appears only twice in the Credo which contains 274 bars, the conclusion seems inescapable that Josquin deliberately determined the various numbers of statements. Evidence of the justness of this conclusion can be found by comparing the present Mass with Josquin's Missa Ave maris stella. Both works are based on a Gregorian chant, the incipits of which are used in ever-changing melodic and rhythmic shapes. Moreover, both present thd cantus prius factus in the tenor. Contrary, however, to the irregular distribution of the "Gaudeamus" motifs in the various Mass sections and the individual voices, the employment of the "Ave maris stella" incipit is much more balanced. Noble says: "....one senses that in Ave maris stella the exuberance of Gaudeamus has begun to be tamed, even spiritualized."Now the interesting thing about all that, according to the author of Symbolic scores: Studies in the music of the Renaissance, Willem Elders, is that it helps him make the case that this Mass was written to celebrate All Saints' Day - and not, as had been previously (and it seems universally) supposed, written for the Feast of the Assumption - another of the feast days that used Gaudeamus as its Introit.Continuing on, the writer says:
The "Gaudeamus" motif in Josquin's Mass of the same name has more than a purely musical significance. It functions, as I shall now attempt to show, as a sign which refers explicitly to the composer's profession of faith. On the basis of the allegorical meaning of the numbers 6, 14, 2, 5, 7, 4 and 23 (see above), it can be said that the Mass in all probability was intended as an All Saints' Day liturgy and that the application of number symbolism may have been inspired by the Book of Revelation.!
I haven't finished reading his argument - it's a long one, and he believes van Eyck's "Adoration of the Lamb," from the Ghent Altarpiece, is involved! - but it's certainly an interesting one so far. ("4" stands for the Cross - and "23" for the 23 times the sign of the cross is made during the liturgy of the mass.
"Why 23 times?" you ask? "[The] number [is understood to be] a reference to the just in the age of the law (10) and in the age of grace (13). The law is valid for both ages (10+10), that is, the ages of the Old and New Convenant, while faith in the threefold God (+3) is an added characteristic for the New Testament.")
Now I ask you: who doesn't like stuff like this? In any case, it's absolutely a perfect tale for a blog like this one; the amazing part played by Gregorian chant in the history of the West!
There's quite a lot at this Wikipedia page about Missa Gaudeamus, too.