This page gives some basic details about the Reflection Glossiness render element and how it is used in compositing.
The Reflection Glossiness Render Element stores the image's reflection glossiness as a grayscale image that represents the degree of Reflection Glossiness set for materials in the scene. White areas have the most reflection glossiness, while dark areas have little.
The amount of reflection glossiness for a material is set by its Reflection Glossiness parameter. For example, V-Ray Material (VRayMtl) has a Reflection Glossiness parameter, and Skin Material (VRayMtlSkin) has parameters for primary and secondary reflection glossiness.
UI Path: ||Toolbar|| > V-Ray Menu icon > Render > VRayRenderElement > Type: Reflection Glossiness
The Reflection Glossiness element has no controllable properties.