Gary graduated from Rapid City High School in 1961. He obtained a Bachelor of Arts degree in government and history in January 1965 and a Master of Arts degree in May 1968, both from the University of South Dakota. He served as instructor of history and government at Upper Iowa University, Fayette, Iowa, from 1965 to 1967. He was an Assistant Professor of History at Northern State College, Aberdeen, South Dakota, from 1967 to 1973. He graduated from the School of Law, University of South Dakota in December 1975. He was admitted to practice as an attorney in South Dakota on February 17, 1976.  

He served as Tripp/Todd County Deputy States Attorney from 1976 to 1981. He also was an associate attorney of the Tobin Law Office, Winner, South Dakota, from 1976 to 1981. He served a term as President of the Rosebud Bar Association in south-central South Dakota.  

In 1981, he moved his law practice to Lake Andes, South Dakota, where he was also licensed as an abstractor and managed the Charles Mix County Abstract Company, where he had an opportunity to practice real estate law from 1981 until 1990.  

He served as Law Trained Magistrate for the First Judicial Circuit Court of South Dakota from 1982 to 1983. He was also in private practice from 1981 until 1990 in Lake Andes. From 1984 through 1989, he served as Charles Mix County States Attorney. Because of his practice on or near Indian Reservations, he also has had a substantial exposure to and practice in Indian Law. He served as a member of the Board of Directors of Dakota Plains Legal Services Corporation. In addition, he served as a member of the Andes Central School District School Board. Gary graduated from the Career Prosecutors Course sponsored by the National District College of District Attorneys in June 1989.  

In 1990, he moved to Sioux Falls, where he practiced at the Galland Legal Clinic until August 1999. While there, he wrote virtually all of the office briefs for the South Dakota Circuit Courts, South Dakota Supreme Court, Federal District Court and the Federal Eighth Circuit Court of Appeals. He has participated as legal counsel in over 1,000 DWI cases.