Gain is determined by output/input, and in this case, is also dependent on the level of applied reverse voltage. Defining “gain” as LED current/Optocoupler current, the following graph illustrates gain at two different levels of Vrwm.

For example, at Vrwm = 1KV, and I(LED) = 30mA, Gain = .00077 = (23uA)/(30mA). Similarly, at Vrwm = 4KV, and I(LED) = 30mA, Gain = .00087 = (26uA)/(30mA)