OK, knowing that I am about to embarrass myself and my mother the Latin teacher, here is an approximation of the meaning of the lyrics. Further clarifications are welcome.
-Joe Offer-

GAUDETE

Gaudete, gaudete
Rejoice, rejoice
Christus est natus
Christ is born
Ex Maria virgine, gaudete.
Of Mary the virgin, rejoice
Tempus ad est gratiae hoc quod optabamus
This is the time of grace which we have desired (approximate rendering)
Carmina laetitiae devote redamus.
We give love in return with songs of joy (approximate rendering)
Deus homo factus est naturam erante,
God is made human (I dunno - into an errant nature???)
Mundus renovatus est a Christo regnante.
The world is made new by Christ reigning
Ezecheelis porta clausa per transitor
Ezechiel has gone through the door which was closed (approximate rendering)
Unde lux est orta sallus invenitor.
From whence comes the saving light (less than approximate)
Ergo nostra contio psallat jam in lustro,
Therefore our assembly sings (or plays) now in (?)(approximate rendering)
Benedicat domino sallus regi nostro.
The Blessing of the Lord and the salvation of our king. (approximate rendering)

From Piae Cantiones (1582)

The following caveats and disclaimers apply:
1. Please note that Medieval Latin is a little different from anything most Latin students learn in school. Besides, this is a Finnish rendition of Medieval Latin. They wrote it in a sauna, I think.
2. I did win theAuxilium Latinum award in freshman year of high school, but this is the biggest translation I've done since 1968.
3. I don't know where the text came from, but it sure doesn't look like what I hear on the CD. Somebody, PLEASE check the lyrics in theOxford Book of Christmas Carols or other reputable publication and post corrections.
4. In addition to the Steeleye Span recording, I found a nice recording by the Ex Cathedra Chamber Choir, whoever they are. Their diction is better, but they sing the verses in hemidemisemiquavers, and I can't understand that fast. I'm sure we have the chorus right, though.
5. Nonetheless, it was fun to make a stab at it.
-Joe Offer-