Wouter,
The radius of gyration is also called the radius of inertia. It is a fictitous radius that when squared and multiplied by the mass of a body produces the same moment of inertia as acquired by integrating the elements of a body times the radius squared to each element over the entire length of the body from some reference axis.
(Radius of Gyration = Moment of Inertia/mass) The comment about the carbon mast having 1/4th the radius of gyration of an aluminum mast can only be correct if the carbon mast has very heavy elements near its base and very lightweight elements near its tip. I doubt that this the case.
Bill