Son of the minister of Careston, near Brechin, he became minister of the college church of Glasgow at 30 until his death in 1796. He is said to have preached three times on Sundays and three times during the week. His two-volume Historical Collections relating to the success of the Gospel (Glasgow 1754) is an account of revivals.
John Wesley met him in Glasgow in 1753. They became friends, and in 1755 he preached for Wesley in Newcastle. Wesley described his preaching as close and convincing. He became interested in hymn-singing instead of the (sometimes doggerel) metrical psalms sung in the kirks since the Reformation. When he introduced hymns to his congregation there was some opposition, but he persisted. Gillies told Wesley that he had not had an experience of salvation but always prayed for it. Without much success, Wesley advised Gillies' congregation in 1757 to examine each other's hearts and lives in their weekly meetings.
Ten years later, Wesley reported Gillies as saying: 'In some opinions I do not agree with the Methodists; but I know they are a people of God, therefore I wish them good luck in the name of the Lord.'