It is in LH. (427) Look here for a transcription:

http://forum.musicasacra.com/forum/discussion/9535/st.-gregory-hymns/p1

Other melodies and extra verses available,

http://forum.musicasacra.com/forum/uploads/FileUpload/4e/72c4e34d355b1b8a964cf851ba78f7.pdf

The Hymn is also in Plainsong for Schools (6 verses),



Liber Responsoralis 1895 (6 verses),


and

Antiphonal Monasticum 1934 (6 verses). Melody as in L.H.

Here is the text from the Liturgical Year, this may be the original text of St. Peter Damien,

1. Anglórum jam Apóstolus, Nunc Angelórum sócius,
Ut tunc,
Gregóri, géntibus Succúrre jam credéntibus.

2. Tu largas opum copías, Omnémque mundi glóriam
Spernis, ut inops ínopem Jesum sequáris príncipem.

3. Vidétur egens náufragus, Dum stipem petit Angelus;
Tu munus jam post géminum, Prǽbes et vas argénteum.

4. Ex hoc te Christus témpore, Suæ præfert Ecclésiæ :

Sic Petri gradum pércipis, Cujus et normam séqueris.

5. O Póntifex egregie, Lux et decus Ecclésiæ,
Non sinas in perículis, Quos tot mandátis ínstruis.

6. Mella. cor obdulcántia. Tua distíllant lábia:
Fragrántum vim arómatum Tuum vincit elóquium.

7. Scriptúræ sacræ mýstica Mire solvis ænígmata:
Theórica mystéria Te docet ipsa Véritas.

8. Tu nactus apostólicam Vicem simul et glóriam:
Nos solve culpæ néxibus, Redde polórum sedibus.

9. Sit Patri laue ingénito, Sit decus unigénito,

Sit utriúsque párili Majéstas summa Flámini.
Amen.

1. O Gregory, that once wast the apostle of the Angli, and now art a companion of the angels! protect now, as of old, the nations that believe in Christ.

2. Thou spurnest wealth and riches and all the glory of the world, that so thou, being poor, mayst follow the Lord Jesus, who was poor.

3. An angel presents himself to thee, in the garb of one that
was shipwrecked, and asks an alms; thou first makest him a double gift, and then thou givest him a silver vase.

4. After this, Christ puts thee over his Church, for thou didst imitate the virtues, and now thou hast the honours, of Peter.

5. O excellent Pontiff! Light and ornament of the Church! Thou hast so richly instructed us, assist us in our dangers.

6. From thy lips there flows honey that brings sweetness to the heart. Thy. words are more fragrant than the richest perfume.

7. Admirably dost thou solve the obscure figures of sacred Writ. The divine mysteries are taught thee by him that is the very Truth.

8. O thou that hast the office and the glory of the apostles, pray for us, that we may be loosed from the bonds of sin, and obtain the thrones prepared for us above.

9. To the unbegotten Father, and to his only-begotten Son, and to the Spirit of them both be praise and highest kingship. Amen.

It is interesting that the melodies in the L.H. are so poor in comparison to say the L.U., I do find it interesting that the L.H. is supposed to use more authentic (older melodies). But the Dominicans have some of the oldest readable manuscripts and their melodies are similar to the L.U. and in many places usually more elaborate and always more beautiful!

While much has been written about the Urban VIII reformed hymns, at least they can be described as being an academic endeavour... What good can we say about the L.H...