Rulings

• 2015-06-22
  Say you activate Gideon's third ability, then an opponent gains control of him before combat. You may have any of your creatures attack Gideon (since he's still a planeswalker). Then Gideon may block (since he's a creature). He may block any eligible attacking creature, including one that's attacking him. During combat, he behaves as an attacked planeswalker and/or a blocking creature, as appropriate. For example, he deals combat damage to any creatures he's blocking, but he doesn't deal combat damage to any unblocked creatures that are attacking him.
• 2015-06-22
  If you activate Gideon's third ability and then damage is dealt to him that can't be prevented, that damage has 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). If Gideon has no loyalty counters on him, he's put into his owner's graveyard as a state-based action. (As long as he still has indestructible, the marked damage won't cause him to be destroyed.)
• 2015-06-22
  Gideon's third ability causes him to become a creature with the creature types Human 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: Gideon is just a planeswalker type (not a creature type), and Human and Soldier are just creature types (not planeswalker types).
• 2015-06-22
  If Gideon becomes a creature due to his third ability, that doesn't count as having a creature enter the battlefield. Gideon was already on the battlefield; he only changed his types. Abilities that trigger whenever a creature enters the battlefield won't trigger.
• 2015-06-22
  If the creature targeted by Gideon's first ability changes controllers before it has the chance to attack Gideon, the ability will apply to it during its new controller's next turn.
• 2015-06-22
  If Gideon can't be attacked, perhaps because he has left the battlefield before the creature's controller's next combat, the creature targeted by Gideon's first ability can attack you or another planeswalker you control, or its controller can choose to have it not attack at all.
• 2015-06-22
  Gideon's first ability causes a creature to attack him if able. If, during its controller's declare attackers step, that creature is tapped, is affected by a spell or ability that says it can't attack, or hasn't been under its controller's control continuously since that player's turn began, then that creature doesn't attack. If there's a cost associated with having that creature attack, its controller isn't forced to pay that cost. If he or she doesn't, the creature doesn't have to attack.