Did Church Fathers other than Saint Gregory the Great teach Purgatory? You betcha.
The Acts of Paul and Thekla, Clement of Alexandria, Tertullian, St Cyprian, St Basil, St Gregory Nyssa, St Ephraem, St Jerome, and St Augustine - all taught purgatory.
Remember, the Jews pray for the dead and the early Church carried over this OT practice.
Here's Saint Augustine on purgatory:
"And it is not impossible that something of the same kind may take place even after this life. It is a matter that may be inquired into, and either ascertained or left doubtful, whether some believers shall pass through a kind of purgatorial fire, and in proportion as they have loved with more or less devotion the goods that perish, be less or more quickly delivered from it. This cannot, however, be the case of any of those of whom it is said, that they 'shall not inherit the kingdom of God,' unless after suitable repentance their sins be forgiven them. When I say 'suitable,' I mean that they are not to be unfruitful in almsgiving; for Holy Scripture lays so much stress on this virtue, that our Lord tells us beforehand, that He will ascribe no merit to those on His right hand but that they abound in it, and no defect to those on His left hand but their want of it, when He shall say to the former, "Come, ye blessed of my Father, inherit the kingdom," and to the latter, 'Depart from me, ye cursed, into everlasting fire.'"
Saint Augustine, Enchiridion, 69