The letters of Gregory the Great do exist in English, as they were translated, complete, a few years ago. Sadly none of us have access to them, unless we have $150 to spare. But today the question was raised of just where a certain letter of Gregory’s can be found.
In various online sources, apparently — the questioner didn’t specify — a reference is present to “lib. vii, ep. 64”. Yet if you look in the Patrologia Latina 77, and go to the back, there is a list of all the letters, and book 7 only has 45 letters.
An article here on Gregory and the Papal supremacy refers to “Fr. Edward Hawarden, Charity and Truth (1728) pp. 233-243″ and gives the following text:
Again, “As to what they say of the See of Constantinople, who doubts but that it is subject to the Apostolic See, as both the Emperor and the Bishop of that See constantly own?” says St. Gregory, in a letter to John, Bishop of Syracusa.2 And writing to the same Bishop concerning the Byzantine primate in Africa: “Whereas he owns himself subject to the See Apostolic, when bishops commit a fault, I know not what bishop is not subject to it. But when there is no fault to be punished, by way of humility, all are equal.”3
 Lib. 7, Ep. 64, p. 1348, D.
 Lib. 7, Ep. 65, p. 1349, AB.
Hmm. So … to what edition does this refer?
I suppose it would be prudent to check the transcription against the original. This is here, where footnote 2 is (30) and the Latin is given:
(30) Nam de Constantinopolitana ecclesia quod dicunt, quis eam dubitet Sedi Apostolicae esse subjectum? Quod et piissimus Dominus Imperator, et Frater noster eiusdem Civitatis episcopus assidue profitentur. L. 7. Epist. 64. p. 1348. D. Nam episcopi nomen proprium in melioris notae codicibus MSS. deest. Vide Nat. Alex. de Scriptis S. Gregorii. Saec. VI. Cap. iv. Art. xvi.
A search on the Latin brings up this link to the NPNF-212, where the reference is given to Lib. IX, Ep. 12. Doubtless the standard numeration has changed since that rather remote day.