On 6 June 2004, while reporting from Al-Suwaidi, a district of Riyadh, Saudi Arabia, Gardner was seriously injured in an attack by al-Qaida gunmen, which left him partially paralysed in the legs.

His father and mother, Robert Neil Gardner (1922–2010) and Evelyn Grace Rolleston (1923–2014), were both diplomats,[1] and when he was six he moved from the UK to the Hague in the Netherlands.

In 1951, while second secretary at the British Embassy in Czechoslovakia, his father was expelled from the country for espionage activities after an incident in a prohibited military area where he was shot at.

[7] On 30 September 1984, he transferred from the general list to the 4th Volunteer Battalion, the Royal Green Jackets, as a second lieutenant (on probation) and was given seniority from 23 May 1984.

[11] He returned to the Territorial Army on 24 April 1997, serving in the Educational and Training Services Branch of the Adjutant General's Corps.

[15] On 6 June 2004, while reporting from Al-Suwaidi,[16] a district of Riyadh, Saudi Arabia, notorious for extremism, Gardner was shot six times and seriously injured in an attack by al-Qaida gunmen.

[18] Despite his injury, he still frequently reports from the field including places like Afghanistan[19] and Colombia[20] but usually comments on top stories from a BBC studio.

During an expedition to Papua New Guinea, they sought the elusive birds-of-paradise (a life-long ambition of Gardner's as an experienced birdwatcher), including the King of Saxony.

[23][24] In March 2012, Gardner pulled out of hosting the Counter-Terrorism and Specialist Security Awards (CTSS) amid concerns that this would compromise the BBC's impartiality.

[25] In June 2018, Gardner chaired the keynote stage across the first two days of the security industry event IFSEC International, taking place in the ExCeL exhibition centre in London.

[26] In 2016, Gardner appeared on Channel 4's coverage of the Paralympic Games in Rio de Janeiro in his role as President of the GB Ski Club.

The documentary focuses on recent gains by Ukrainian forces in the Russo-Ukrainian War, and how it could provoke President Putin into the use of tactical nuclear weapons.

[28] Gardner's Sunday Times bestseller Blood and Sand (ISBN 978-0-553-81771-3), describing his 25 years of Middle Eastern experiences, was published by Transworld in 2006.

Gardner's bestselling first novel, Crisis, featuring the fictional SBS officer-turned MI6 operative Luke Carlton and a ruthless Colombian drug lord, was published in June 2016.