Gregory John Hartmayer was born in Buffalo, New York, one of four children of John and Sally Hartmayer. He was raised in nearby Tonawanda, where he received his early education at St. Amelia School. He graduated from Cardinal O'Hara High School in 1969.
In 1969, Hartmayer joined the Order of Friars Minor Conventual, commonly known as the Conventual Franciscans, at the St. Joseph Cupertino Friary in Ellicott City, Maryland. He took his simple vows as a Conventual Franciscan friar on August 15, 1970, before making his solemn profession on August 15, 1973. He also studied at St. Hyacinth College and Seminary in Granby, Massachusetts, where he obtained a Bachelor of Science degree in philosophy in 1974. From 1974 to 1975, he taught at Archbishop Curley High School in Baltimore. He then returned to New York to study theology at St. Anthony-on-Hudson Seminary in Rensselaer, receiving a Master of Theology degree in 1979.
Bishop Hartmayer was ordained to the priesthood by Bishop Howard J. Hubbard on May 5, 1979, at the Cathedral of the Immaculate Conception in Albany, NY. He then returned to Archbishop Curley High School, where he served as a guidance counselor and teacher (1979-1985) and principal (1985-1988). In 1980, he earned a Master of Arts degree in pastoral counseling from Emmanuel College in Boston. He served as principal of his alma mater of Cardinal O'Hara High School in Tonawanda from 1988 to 1989, when he became principal of St. Francis High School in Athol Springs. He received a Master of Education degree in Secondary Catholic School Administration from Boston College in 1992.
Following a three-month sabbatical at St. Patrick Seminary in Menlo Park, California, Bishop Hartmayer briefly served as an instructor at John Carroll Catholic High School in Fort Pierce, Florida, in 1995. In August of that year, he was named pastor of St. Philip Benizi Church in Jonesboro, Georgia. He became pastor of St. John Vianney Church in Lithia Springs, in July 2010.
On July 19, 2011, Bishop J. Kevin Boland of the Diocese of Savannah announced his retirement and Pope Benedict XVI appointed Hartmayer as the bishop of the diocese. His consecration took place on October 18, 2011, at the Cathedral of St. John the Baptist in Savannah, Georgia.