#prevention effects

The controller of Gisela is irrelevant here beyond which of her abilities applies to the damage being dealt, so she could be yours or another opponent's and the result would be the same.

So what is the result here? Unfortunately, it's the worst case scenario for you, or at least it's likely to be. Both Gisela's ability and Ria's ability affect the combat damage that the creature would deal to your opponent. When multiple replacement or prevention effects want to modify the same combat damage event, then the player being damaged (or the controller of the creature or planeswalker being damaged) gets to choose which effect gets applied first. In this case, they'll almost certainly choose to prevent the damage with Ria's ability first, leaving nothing for Gisela to double.

Prevention effects, like from protection, and replacement effects, like from Soul-Scar Mage are handled at the same time. One doesn’t take priority over the other. So, you won’t kill True-Name Nemesis here unless your opponent is feeling generous.

When Anger of the Gods (or any other source you control) would deal noncombat damage to True-Name Nemesis, both the prevention effect of True-Name Nemesis’s protection ability and Soul-Scar Mage’s replacement effect want to modify the damage event. As the controller of the affected permanent (e.g. the permanent that would be dealt damage), your opponent chooses which replacement or prevention effect to apply to the damage event first. If they’re smart, they’ll choose to apply the protection effect first, which prevents the damage. Since there’s no longer any damage being dealt, there’s nothing for Soul-Scar Mage to replace with -1/-1 counters.

Whenever multiple replacement and/or prevention effects want to apply to the same instance of damage, the player being damaged or the controller of the creature being damaged chooses the relative order in which those effects are applied. This will usually result in the lesser amount of damage being dealt.

In the situation you describe, since Gisela is the creature being dealt damage and you control Gisela, you choose the order in which the two effects are applied. Ideally, you’ll want to halve and round down the damage with Gisela’s ability before Dictate doubles the damage again, resulting in a total of 2 damage.

Eidolon’s trigger from you casting Deflecting Palm will resolve before Deflecting Palm does, so that damage is going to be dealt to you.