I've been looking for the correct text and tune for this song for a long time. I finally found it in The Shorter New Oxford Book of Carols. The text still wasn't completely right, because the letters "c" and "j" were used improperly. I substituted "t" and "i" for the offending letters, where appropriate. Dick, I hope you'll substitute this for the flawed text in the database. I'm quite certain this text is the correct one.
-Joe Offer-

GAUDETE
From Piae Cantiones (Finland, 1582)

Gaudete, gaudete!
Christus est natus
Ex Maria virgine:
Gaudete!

Tempus adest gratiae,
Hoc quod optabamus
Carmina laetitiae
Devote redamus.

Deus homo factus est
Natura mirante,
Mundus renovatus est
A Christo regnante.

Ezechielis porta
Clausa pertransitur
Unde Lux est orta
Salus invenitur.

Ergo nostra contio
Psallat iam in lustro,
Benedicat Domino
Salus Regi nostro.

Translation (from The Shorter New Oxford Book of Carols):

Rejoice! Rejoice! Christ is born of the Virgin Mary; Rejoice!

The time of grace has come for which we have prayed;
Let us devoutly sing songs of joy.

God is made man, while nature wonders;
The world is renewed by Christ the King.

The closed gate of Ezekiel has been passed through;
From where the light has risen (the East), salvation is found.

Therefore, let our assembly sing praises now at this time of purification;
Let it bless the Lord: greetings to our King.

Notes (also from The Shorter New Oxford Book of Carols): No music is given for the verses in Piae Cantiones. They derive from the medieval Bohemian song 'Ezechielis Porta,' which Finnish clerical students would have encountered in Prague and which shared a tune with a Czech vernacular Christmas song that still survives. Finno, the editor of Piae Cantiones, was probably responsible for the refrain. It adapts the words of one of the medieval verses to the music which, in various forms, was sung throughout Lutheran Germany to Luther's single-stanza grace before meals, 'Danket dem Herren.'
Verse 3 refers to the eastern gate of the city in Ezekiel's vision (Ezekiel 44:2). The gate is a traditional symbol of Mary as perpetual virgin.

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Click to play

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ABC format:

X:1
T:Gaudete!
M:2/2
Q:1/4=240
K:G
A4A4|G4A2B2|c4c2B2|-B2A2G4|G4G4|A4B4|-B2A2G4|
A2B4A2|^G4A4|-A4c2c2|c2d2e2=f2|e4c2A2|B2c2A4|
A4c2c2|c2d2e2=f2|e4c2A2|B2c2A4|A7/2||