Judge Gaertner graduated from Roosevelt in June 1955. He played varsity baseball and was a delegate to the Model United Nations Assembly in New York and Washington, DC.

Judge Gaertner then attended St. Louis University receiving his law degree in 1961. He went into private practice, then joined the City Counselor’s office for the City of St. Louis as an Assistant City Counselor and was later promoted to Associate City Counselor. In August 1968, Mayor Alphonso Cervantes appointed him to head the department as City Counselor. In this capacity, Judge Gaertner represented the City in a variety of legal roles, issuing legal opinions, managing civil litigation, and prosecuting violators of various city ordinances.

In 1969, Governor Warren E. Hearnes appointed Judge Gaertner a Circuit Court Judge for the City of St. Louis. During the next 16 years, Judge Gaertner presided over a thousand criminal and civil trials, along with approving numerous criminal plea agreements and out of court civil case settlements. Judge Gaertner ran the Juvenile Court from 1971 to 1975 and was elected by his fellow judges to the Presiding Judge in 1983.

1n 1985, Governor John R. Ashcroft appointed Judge Gaertner to sit on the Missouri Court of Appeals for the Eastern District of Missouri. This district handles appeals from twenty-five Eastern Missouri counties. Its fourteen judges hear about 50% of the appellate cases in the State. He served in this position until his retirement in 2007 when he reached the mandatory retirement age of seventy. Judge Gaertner presently serves as Senior Judge under the Missouri Supreme Court. In 1993, U.S. Representative Richard A. Gephardt recommended him as a United States District Court Judge of the Eastern District of Missouri.

Judge Gaertner was a member of the Policeman and Fireman’s Fund, better known today as The Backstoppers. He is on the Board of Directors and was its President in 1989-90. He was Vice President of George Khoury International Leagues, which runs youth baseball leagues. Judge Gaertner is a member of the American, Missouri and St. Louis Bar Associations. He served as Chair of the Commission on Retirement, Removal and Discipline of Judges from 1989-1990. He is a member of the Phi Delta Phi fraternity.

The St. Louis Grand Jury Association gave him its Judiciary Award in 1972. The St. Louis Junior Chamber of Commerce recognized him as an Outstanding Young St. Louisan also in 1972. He was the Khoury International Leagues’ Man of the Year in 1975.

Judge Gaertner served in the Coast Guard Reserve from 1954 to 1961 holding the rank of Petty Officer.

Judge Gaertner’s wife, Maureen, a schoolteacher, passed away in 2018. They have three children and twelve grandchildren and one great grandchild. One of their sons, Gary M. Gaertner Jr., currently serves as a Judge on Missouri Court of Appeals for the Eastern District of Missouri and another son, Mark, is the Municipal Judge for Chesterfield and Ladue.