Radius of gyration is indirectly to the parallel axis theorem. The parallel axis theorem gives the moment of inertia ______________ to the surface of considerence.

Parallel

Parallel axis for any area is used to add the two mutually perpendicular moment of inertias for areas. It gives a moment of inertia perpendicular to the surface of the body. That is the moment of inertia perpendicular to the surface in considerence.