Geens received a Master in Law from the Katholieke Universiteit Leuven as well as a licentiate of economy.
In 1961 Geens, together with Frank Swaelen and Leo Tindemans, was part of a think tank of the CVP.
In 1974 in a government led by Tindemans, Geens became Secretary of State for the Budget and Science.
He remained Minister of Finance under Paul Vanden Boeynants and in the first two coalitions led by Wilfried Martens.
Gaston Geens was seventy when he died at home due to cardiac arrest.