He was a Senior Legal Officer at the International Criminal Tribunal for the former Yugoslavia, where he was the senior adviser to the Trial Chamber in the Milošević case. He has consulted for a range of governments, including the Australian federal government and Victorian government departments, as well as the United Kingdom Foreign and Commonwealth Office. He has also consulted for international organisations and NGOs, writing briefing papers and providing training. Gideon has been pro bono legal counsel for accused war criminal, Radovan Karadžić and has appeared and provided expert advice on extradition and other matters. He has published many books, book chapters and articles on international law, criminal law, evidence and justice.