I came across the Orthodox origins of "Gregorian masses" last night, in the Evergetinos:
Hypothesis XXX: Not just for the living, but also for those who have reposed, oblations provide great aid.
From St. Gregory the Dialogist:
11. Believe, then, Peter, that such things take place among the living, too, even if the same are not at all aware of the oblations offered for them. From this, it is also demonstrated before all that the souls of the dead are unimaginably benefited from oblations on their behalf, as long as they have not fallen to some unforgivable sin. A story told about a certain Justus, which is elsewhere recorded, shows the benefit which Divine oblations effect for the dead.
12. Now, this Justus was a monk in our monastery, while I was still living there and before I became a Bishop. Once Justus secretly acquired three coins, despite the rules of the monastery. This was kept secret until the time of his death. And it was on this account that he was deemed unworthy of a funeral service, even though he had repented for his fall before he died. His body, without any prayer whatsoever, was thrown in the manure pile, along with those three ill-gotten coins. And I commanded that all of thebrotherhood should condemn Justus and his coins, saying: “Thy money perish with thee” (Acts 8:20). This was declared by all of the brothers in one voice.
13. After thirty days had elapsed, I felt sorry for the man and ordered that every day, for thirty days, there be celebrated the Bloodless Sacrifice on his behalf, that he might be redeemed from among those in torment. On the thirtieth day, the dead man appeared, in his sleep, to his brother according to the flesh, Copiosus, who asked the departed man: “How are you faring in the other world?” The departed Justus replied: “Until today, I suffered greatly. However, I am now very well.”
14. The brother of the departed monk, Copiosus, went to the monastery and recounted the dream. And counting the exact number of days from Justus’s death until the day on which his brother had this vision, it was confirmed that the report of the refreshment given to the departed’s soul occurred on the day of the completion of the thirtieth Liturgy. It is abundantly clear, then, that the departed brother was delivered from his torments, thanks to the power of the oblations for deliverance; that is, the power of Divine Liturgies.
- The Evergetinos: A Complete Text, CTOS edition, Vol. 4, pp. 354-355