I understood that what I suggested isn't what you asked for, but it wasn't clear why you asked for that. I assumed that you wanted the warning for the case where the gain was not what you last set it to (as in, maybe you had used the transmitter for something else). I assumed that if you heard no warning, you would fly without having to reset the gain. In that case, my scheme accomplishes the goal, and perhaps more.
In any event, my circuit and program could easily be modified to do exactly what you asked for. It would be bigger, heavier and more expensive though...with the switch and buzzer.
My circuit was built with a PIC12F683, a bypass capacitor and a resistor to guard against shorts on the output, so it is very simple. Adding the buzzer and switch would also require a couple more resistors and a transistor. Nothing very significant, but much heavier.
Mike