Instant
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Gideon’s Triumph can be cast after combat damage has been dealt. In this case, the target player chooses one or two creatures that attacked or blocked this turn and have survived combat damage.
A creature that was put onto the battlefield attacking didn’t attack, so it can’t be sacrificed.
A creature that attacked and was removed from combat (such as by Spires of Orazca) still attacked, so it can be sacrificed.
Gideon’s Triumph targets the player, not the creature or creatures to be sacrificed. A creature with hexproof may be sacrificed this way.
Whether you control an appropriate planeswalker is checked only as the spell resolves.
Controlling more than one planeswalker of the appropriate type won’t have any more of a benefit than just controlling one.