Glory to God in the Highest is a prayer we pray during the Catolic Mass.

Glory to God in the Highest
Glory to God in the highest,
and on earth peace to people of goodwill.
We praise you;
we bless you;
we adore you;
we glorify you;
we give you thanks for your great glory,
Lord God, heavenly King,
O God, almighty Father.
Lord Jesus Christ, only Begotten Son.
Lord God, Lamb of God, Son of the Father,
you take away the sins of the world, have mercy on us;
you take away the sins of the world, receive our prayer;
you are seated at the right hand of the Father, have mercy on us.
For you alone are the Holy One, you alone are the Lord,
you alone are the Most High, Jesus Christ,
with the Holy Spirit,
in the glory of God the Father.
Amen.
Mass is a prayer of praise and worship. During our Catholic Mass, we pray, Glory to God in the Highest. We have just shown sorrow for our sins and asked for God’s mercy as we prayed the Confiteor and Lord Have Mercy. Now, we give God glory to God the Father, to His Son, our Lord Jesus Christ, with the Holy Spirit. We acknowledge that there is only one God and He is Almighty. We lift our voices with those in Heaven to praise and worship God.
If you are looking for more prayers to help you grow closer to our Lord you can find more in the Prayers section. There is also a section focused on Mass Prayers.
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is it ok to alter the prayer in song. our church adds stuff
That is a good question. I have heard different versions of prayers, so there may not be one set text that is used for a particular prayer, but different ones that can be used. You could asl ask your priest about this.