L1 Judge here. If I were called to the situation you described in your first post, I’d rule that player B’s explanation is correct: Player A putting their Gisela in the graveyard indicates that they chose to apply the damage doubling effect first, which is a legal choice for them to make. Player B’s choice of applying the damage reduction effect first is also legal, so there’s nothing to do here: Player A’s Gisela is dead, Player B’s Gisela is alive, and they can continue with the game.
Note that it doesn’t matter whether Player A knew that they could choose in which order they apply the replacement effects. Player B gets an advantage for having a superior knowledge of the rules, but that’s how competitive Magic works.