What will happen is the focus of the gain cursor will change from one plot to another. That is, when you first use it the trace that the gain cursor attaches itself to is the green one. If you press the tab key once while the left mouse button is down the gain cursor switches to the yellow trace (peak power) and if you press it again it switches to the red trace (geometric mean of the two). Note that it will only change focus while the left mouse button is down and you are using the gain cursor. There is a minor issue of the crosshair not actually updating correctly on some versions of windows that needs resolving but it you move the mouse after pressing the tab key it should show up on the next trace.