Calvin, a reformer and no lover of popes called Gregory the "last great Pope"
Good things about Gregory:
He became pope during an epidemic of the plague in Rome which killed his predecessor and continued to care for the poor and dying after he was elected pope.
He believed wealth belonged to the poor and the church was only it's steward. He believed the church should not be in the business of making a profit and tried to clean up the correption in the church.
When the Lombards invaded Rome and he couldn't get any help from the Eastern Emperor or the civil government he rallied the people to defend the city, negotiated a peace with the Lombards and arranged for the church to care for war refugees.
He sent missionaries to Britian which led to the spread of Christianity there. Although what led him to do that seems a little odd--he thought the blond blued eyed slaves from Britian looked angelic to him and belonged in heaven.
He founded a school for the training of church musicians, and Gregorian chant (plainchant) is named for him.
He organized the scripture readings for Sundays and wrote accompanying prayers that the church used for the next 1300 years.
He understood Christ's mandate that those who wished to be great must be the "slave of all" and taught that church leaders, from monks and priests all the way to the Pope were to be a servant of the people.
Two not so good things - He had some weird ideas about purgatory that now even the Roman Catholic Church would probably disavow. And he started that pesky rumour about Mary Magdalene being a prostitute!
But hey - nobody's perfect and certainly the church is better off for his being here.