After graduating from high school, Gregg enlisted in the military in 1945 and received training as a cryptanalyst, but did not finish in time to be posted overseas.

[2] In 1979 Gregg, his career in the CIA stalled by Stansfield Turner's new regime, took a post in the United States National Security Council (NSC) as Asia policy and intelligence matters specialist.

[2][6] Gregg, while at the Korea Society during President George W. Bush's administration, helped establish a program "of bringing North Koreans for information technology training and other issues" to Maxwell.

[8][9] Gregg also appeared on PBS News Hour to discuss the agreement with Balbina Hwang, visiting professor at Georgetown University and a Korea specialist at the State Department during the last Bush administration.

In 2014, Gregg published Pot Shards: Fragments of a Life Lived in CIA, the White House, and the Two Koreas ISBN 978-0990447115, a memoir.