After enlisting in the New Zealand Army in 1971, Gardiner attended the Royal Military College, Duntroon in Canberra, graduating as a Bachelor of Arts in 1975, having majored in economics and geography. He then entered the Royal New Zealand Army Ordnance Corps, and later served in the Army Office, Canberra. In December 1990 he was appointed commanding officer of the 1st Base Supply Battalion, with the rank of lieutenant colonel. He was posted to the United Nations Operations in Somalia, Headquarters of the Peacekeeping Force, as deputy chief logistical officer in 1994, and the following year he returned to New Zealand and was promoted to colonel, assuming the position of Chief of Staff, Support Command. After completing a degree in human resource management, and attending the Australian College of Defence and Strategic Studies in Canberra, Gardiner was appointed director of resource policy and plans at the New Zealand Defence Force headquarters in late 1997. He was promoted to brigadier in December 1998 and appointed deputy Chief of Staff, but was appointed Land Commander a week later. He was reappointed deputy Chief of Staff in December 1999 and the following July he received a 12-month posting as chief military observer with the United Nations Transitional Administration in East Timor. He was appointed Head New Zealand Defence Staff, Canberra, in 2001.