Question: What is a Gregorian Mass?
Answer: Pope Saint Gregory I (the Great; 590-604) is generally credited with the final formation of the Canon and the surrounding ceremonies of the Roman Mass. Thus, in a general sense, the traditional Mass may be referred to as the "Gregorian Mass." (The adjective "Tridentine" misdates our Mass by about a thousand years!)
More specifically, though, the term "Gregorian Mass" usually refers to an unbroken series of thirty Masses offered for a soul presumed to be in Purgatory. This thirty day "Trental" is attributed to Gregory the Great who offered it for a monk who died repentant after breaking his vow of poverty. The monk is said to have appeared to Gregory first in torment, and then in heavenly bliss after the last Mass. In medieval England, the Trental consisted of the thirty most significant feasts of the liturgical year, and was based on an unlikely claim that it was first required for Pope Gregory's mother. (Unlikely, because she was a woman of great holiness, occasionally referred to as the "Episcopa" during Gregory's pontificate.)