If multiple replacement effects would modify how damage would be dealt, the player being dealt damage (or the controller of the permanent being dealt damage) chooses the order in which to apply those effects.

If damage dealt by a source you control is being divided or assigned among multiple permanents an opponent controls or among an opponent and one or more permanents they control simultaneously, divide the original amount and double the results. For example, if you attack with a 5/5 creature with trample and your opponent blocks with a 2/2 creature, you can assign 2 damage to the blocker and 3 damage to the defending player. These amounts are then doubled to 4 and 6 damage, respectively. You can't double the damage to 10 first and then assign 2 to the creature and 8 to the player.

Gisela doubles damage dealt to opponents and permanents your opponents control from any source, including sources controlled by those opponents.