The "Gradual side tilt angle change" is an angle offset onto the side tilt angle setting. This value plus the side tilt angle is the final tool axis orientation, when the machining is finished. So at the first cut, the tool axis is tilted with just a side tilt angle. At the last cut the tool axis is tilted with a side tilt angle plus the gradual side tilt angle. Every new slice the axis will be tilted more and more.
E.g. the side tilt angle is set by user to 5 degrees. The gradual side tilt angle is set to 10 degrees. That means, at the beginning of the toolpath the side tilt angle will be 5 degrees, at the end of the toolpath, the side tilt angle will be 5 + 10 = 15 degrees. At the halfway of the toolpath, the side tilt angle will be 5 + 10 * %50 = 10 degrees.