At a time when the bishops, mostly simoniacal, were the dependants of lay princes, he strove with such constancy to defend the liberty of the Church (Collect) that, as we are assured, no pontiff since the time of the apostles undertook more labours for her or fought more courageously lor her independence.
While he was saying Mass, a dove was seen to come down on him: the Holy Ghost thereby bore witness of the supernatural views that guided him in the government of the Church. Forced to leave Rome, he died in Salerno in 1085, saying those words, the first of which are from Ps. xliv: "I have loved Justice and have hated iniquity: that is why I die in exile."
Following the example of St. Gregory, let us overcome with courage all adversities (Collect).
Statuit ei Dominus testamentum pacis, et principem fecit eum: ut sit illi sacerdotii dignitas in aeternum. * Memento Domine David et omnis mansuetudinis ejus.
The Lord made to him a covenant of peace, and made him a prince; that the dignity of the priesthood should be to him for ever. * O Lord, remember David: and all his meekness.
(Ecclus. 45:30 and Psalm 131:1. From the Introit at Mass).
Deus in te sperantium fortitudo, qui beatum Gregorium, Confessorem tuum atque Pontificem, pro tuenda Ecclesiae libertate, virtute constantiae roborasti: da nobis, ejus exemplo et intercession, omnia adversantia fortiter superare.
O God, the strength of all that put their trust in Thee, who for the defence of the liberty of Thy Church didst fill blessed Gregory, Thy confessor and bishop, with the virtue of constancy: grant that, helped by his prayers and example, we, too, may bravely overcome all adversity.
From the Catholic Encyclopaedia: http://www.newadvent.org/cathen/06791c.htm