Cheapest priced copy (The List): $1.29 at Card Kingdom, near mint, their price list as of .
Gideon, Ally of Zendikar 
Legendary Planeswalker — Gideon · Mythic · Battle for Zendikar · #29
+1: Until end of turn, Gideon becomes a 5/5 Human Soldier Ally creature with indestructible that's still a planeswalker. Prevent all damage that would be dealt to him this turn.
0: Create a 2/2 white Knight Ally creature token.
−4: You get an emblem with "Creatures you control get +1/+1."
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
5 printings · $1.29 to $199.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Battle for Zendikar | 29 | Mythic | $1.79 | $4.49 |
| Battle for Zendikar Promos | 29s | Mythic | — | $15.99 |
| The List | BFZ-29 | Mythic | $1.29 | — |
| Regional Championship Qualifiers 2022 | 1 | Mythic | $11.99 | $199.99 |
| San Diego Comic-Con 2016 | 29 | Mythic | — | $18.99 |
4 rulings from Wizards of the Coast
- 25 August 2015If 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).
- 25 August 2015Gideon's first ability causes him to become a creature with the creature types Human, Soldier, and Ally. 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 Human, Soldier, and Ally are just creature types (not planeswalker types).
- 25 August 2015Gideon's first ability doesn't count as a creature entering the battlefield. Gideon was already on the battlefield; he only changed his types. Specifically, your rally abilities won't trigger. (Of course, Gideon's second ability will cause rally abilities to trigger.)
- 25 August 2015If damage that can't be prevented is dealt to Gideon after his first 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 has indestructible, if Gideon has no loyalty counters on him, he's put into his owner's graveyard.