The GradYPos defines the vertical position of the secondary color shading when the type of Fill is set to Radial Gradient.
It is unusual for the GradYPos to be animated, as the position of the gradient shading is not usually changed after it has been defined on the Polyline Properties window or Text Properties window. However, if you wanted to animate the GradYPos, the expression for the GradYPos would need to result in a value between 0 and 100 (0 is at the bottom and 100 is at the top).