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