This lawsuit build on a false premise. Steam doesnt have a price parity clause for other stores. What this lawsuit alleges applies to Steam keys that the developer generates through Steam. If the developer lists those keys for sale at a price lower than what the game is listed for on Steam, then the price of the Steam Store purchase price must match it, so that people visiting the store page on Steam get the same discount. It doesn't matter if you list your game on GOG and discount it there.