Keller earned a history professor agrégé degree in 1972 at Paris West University Nanterre La Défense.
He graduated from the École nationale d'administration (ENA) in 1979 and began his diplomatic career by joining the Ministry of Foreign and European Affairs (France) (MFA).
His position took him to several French embassies abroad, which led him to hold various responsibilities within the United Nations and the Organization for Security and Co-operation in Europe (OSCE).
While at the Ministry of Foreign and European Affairs in Paris, Keller was head of the Methods, Training and Computing Department from 1986 until 1989.
Gabriel Keller joined the Institut des hautes études de défense nationale (IHEDN) in 1994, graduating in 1995.