Today is the commemoration of Saint Gregory the Great, who was the Bishop of Rome from 590 to 604.
To musicians he is remembered as the author (or perhaps just compiler) of a great deal of plainchant that came to bear his name: Gregorian Chant.
To Anglicans he is remembered as the one who sent St. Augustine and his team to England, where, based in Canterbury, the Anglo-Saxons were re-evangelized and the Church there reinvigorated.
To Roman Catholics he is remembered as one of the 36 ‘Doctors of the Church’.
To the Eastern Orthodox he is remembered as the author of the Dialogues, chronicling the lives and miracles of various early Saints, especially including Saint Benedict.
To many Bishops he is remembered as the author of the Liber regulae pastoralis – for centuries the definitive book on how a Bishop is to order his life.
To the Reformer John Calvin he is remembered as “the last good Pope.”

If we to commemorate him in a Communion service today, there are two main options for Collects and Lessons.

Of a Teacher of the Faith

Almighty God, you gave your servant Gregory the Great special gifts of grace to understand and teach the truth revealed in Christ Jesus: Grant that by this teaching we may know you, the one true God, and Jesus Christ whom you have sent; who lives and reigns with you and the Holy Spirit, one God, for ever and ever. Amen.

Proverbs 3:13-26; Psalm 119:89-106; 1 John 1:1-10; Matthew 13:47-52

Of a Pastor

O God, our heavenly Father, you raised up your faithful servant Gregory the Great, to be a bishop and pastor in your Church and to feed your flock: Give abundantly to all pastors the gifts of your Holy Spirit, that they may minister in your household as true servants of Christ and stewards of your divine mysteries; through Jesus Christ our Lord, who lives and reigns with you and the Holy Spirit, one God, for ever and ever. Amen.

Isaiah 6:1-8; Psalm 71:17-24; 1 Peter 5:1-11 or Acts 20:24-35; Matthew 24:42-50

What one would do is choose either of these sets, and stick with them wholesale; don’t mix and match between them.  Decide how you’re going to commemorate St. Gregory, identify what aspect of his legacy and sainthood you wish to highlight to the congregation, and choose the Propers (Collect & lessons) accordingly.

A further recommendation of this Customary, because this is an optional commemoration and not a Prayer Book “red letter day”, would be to use two readings (plus Psalm) instead of three.  Remember also that you can omit the Nicene Creed, which the rubrics require only for Sundays and Major Feast Days.

And, of course, there’s nothing stopping you from reading and praying an Antecommunion service on your own – that is, going through the Communion liturgy up to the Offertory and ending it there with the Lord’s Prayer!