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Whenever you attack with two or more non-Gideon creatures, put a +1/+1 counter on each of those creatures.
[+2]: Until end of turn, Gideon, the Oathsworn becomes a 5/5 white Soldier creature that's still a planeswalker. Prevent all damage that would be dealt to him this turn. (He can't attack if he was cast this turn.)
[−9]: Exile Gideon, the Oathsworn and each creature your opponents control.
A non-Gideon creature is a creature that isn’t also a Gideon planeswalker. For example, Gideon’s Company is a non-Gideon creature.
Any attacking Gideon creatures won’t get a +1/+1 counter, but as long as two or more attacking creatures aren’t Gideons, those attacking creatures each get a +1/+1 counter.
Gideon’s first loyalty ability causes him to become a creature with the creature type Soldier. He remains a planeswalker with the planeswalker type Gideon. (He also retains any other card types or subtypes he may have had.) Each subtype is correlated to the proper card type: planeswalker is only a type (not a creature type), and Soldier is just a creature type (not a planeswalker type).
Gideon’s first loyalty ability doesn’t count as a creature entering the battlefield. Gideon was already on the battlefield; he only changed his types.
If Gideon becomes a creature the same turn he enters the battlefield, you can’t attack with him or use any of his {T} abilities (if he gains any).
If Gideon has 9 loyalty as you activate his last ability, he’ll be put into your graveyard and won’t be exiled.
If damage that can’t be prevented is dealt to Gideon after his first loyalty ability has resolved, that damage will have all applicable results: specifically, the damage is marked on Gideon (since he’s a creature) and that damage causes that many loyalty counters to be removed from him (since he’s a planeswalker). Even though he’s also a creature, if Gideon has no loyalty counters on him, he’s put into his owner’s graveyard.
