Gideon’s Triumph targets the player, not the creature or creatures to be sacrificed. A creature with hexproof may be sacrificed this way.

A creature that was put onto the battlefield attacking didn’t attack, so it can’t be sacrificed.

Controlling more than one planeswalker of the appropriate type won’t have any more of a benefit than just controlling one.

Whether you control an appropriate planeswalker is checked only as the spell resolves.

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 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.

Gideon’s Triumph can be cast before combat damage has been dealt. In this case, it can be cast before or after blockers have been chosen.