The Dialogues consists of four books of reflections on the saints of Italy. The second book is on St. Benedict, and it is from this book that the most famous passage in the West comes (Chapter 35):
The man of God, Benedict, being diligent in watching, rose early up before the time of matins (his monks being yet at rest) and came to the window of his chamber, where he offered up his prayers to almighty God. Standing there, all on a sudden in the dead of the night, as he looked forth, he saw a light, which banished away the darkness of the night, and glittered with such brightness, that the light which did shine in the midst of darkness was far more clear than the light of the day. Upon this sight a marvellous strange thing followed, for, as himself did afterward report, the whole world, gathered as it were together under one beam of the sun, was presented before his eyes, and whiles the venerable father stood attentively beholding the brightness of that glittering light, he saw the soul of Germanus, Bishop of Capua, in a fiery globe to be carried up by Angels into heaven.
There are a number of famous hagiographical stories told about St. Gregory himself. One of them, preserved by St. Bede in his Ecclesiastical History (Book II, Chapter I), is the famous story of the puns that first gave him the thought (before he was pope) that a mission should be sent to England:
Nor must we pass by in silence the story of the blessed Gregory, handed down to us by the tradition of our ancestors, which explains his earnest care for the salvation of our nation. It is said that one day, when some merchants had lately arrived at Rome, many things were exposed for sale in the market place, and much people resorted thither to buy: Gregory himself went with the rest, and saw among other wares some boys put up for sale, of fair complexion, with pleasing countenances, and very beautiful hair. When he beheld them, he asked, it is said, from what region or country they were brought? and was told, from the island of Britain, and that the inhabitants were like that in appearance. He again inquired whether those islanders were Christians, or still involved in the errors of paganism, and was informed that they were pagans. Then fetching a deep sigh from the bottom of his heart, “Alas! what pity,” said he, “that the author of darkness should own men of such fair countenances; and that with such grace of outward form, their minds should be void of inward grace.” He therefore again asked, what was the name of that nation? and was answered, that they were called Angles. “Right,” said he, “for they have an angelic face, and it is meet that such should be co-heirs with the Angels in heaven. What is the name of the province from which they are brought?” It was replied, that the natives of that province were called Deiri. “Truly are they De ira,” said he, “saved from wrath, and called to the mercy of Christ. How is the king of that province called?” They told him his name was Aelli; and he, playing upon the name, said, “Allelujah, the praise of God the Creator must be sung in those parts.”
Another famous hagiographical story about St. Gregory, and one that has had more theological influence, was the Miracle of the Salvation of Trajan, the basic elements of which are found in the earliest Life of Gregory. The most famous reference to it is Dante's Purgatorio X, although it was quite well known; St. Thomas Aquinas refers to it a few times, and whenever he does, is always careful not to rule it out. The story goes that St. Gregory passed by Trajan's Forum one day, and started thinking about how the Emperor Trajan had aided and defended a widow in dire straits simply because she asked for help. Weeping at the thought of it, he asked God to deliver Trajan's soul from the pains of hell. Different versions of the story go different ways at this point. According to one version, Trajan in hell is from that point on freed from any suffering or pain (i.e., his entire punishment consists solely of not receiving the Beatific Vision in Heaven). According to another version, God resurrected Trajan long enough for St. Gregory to baptize him, thus freeing him from hell entirely; according to some versions of this version God then rebuked St. Gregory for being presumptuous in thinking that he knew better than God how God's mercy should be applied.