The GradXPos defines the horizontal position of the secondary color shading when the type of Fill is set to Radial Gradient.
It is unusual for the GradXPos 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 GradXPos, the expression for the GradXPos would need to result in a value between 0 and 100 (0 is at the far left and 100 is far right).