+1: Until your next turn, prevent all damage target permanent would deal.
0: Until end of turn, Gideon becomes a 4/4 Human Soldier creature with indestructible that's still a planeswalker. Prevent all damage that would be dealt to him this turn.
0: You get an emblem with "As long as you control a Gideon planeswalker, you can't lose the game and your opponents can't win the game."
(18/04/2017)
Gideon's second ability causes him to become a creature with the creature types Human and 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 Human and Soldier are just creature types (not planeswalker types).
(18/04/2017)
If Gideon becomes a creature the same turn he enters the battlefield, you can't attack with him or use any of his abilities (if he gains any).
(18/04/2017)
If you have Gideon's emblem in a Two-Headed Giant game and control a Gideon planeswalker, your team can't lose the game and the opposing team can't win the game. If your teammate controls a Gideon planeswalker but you do not, your emblem's effect doesn't apply.
(18/04/2017)
If damage that can't be prevented is dealt to Gideon after his second 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.
(18/04/2017)
Other circumstances can still cause you to lose the game. You will lose a game if you concede, if you're penalized with a Game Loss or a Match Loss during a sanctioned tournament due to a DCI rules infraction, or if your Magic Online® game clock runs out of time.
(18/04/2017)
No game effect can cause you to lose the game or cause any opponent to win the game while Gideon's emblem is in effect. It doesn't matter whether you have 0 or less life, you're forced to draw a card while your library is empty, you have ten or more poison counters, you're at your Glorious End, your opponent casts a second Approach of the Second Sun, or so on. You keep playing.
(18/04/2017)
Gideon's second ability doesn't count as a creature entering the battlefield. Gideon was already on the battlefield; he only changed his types.
(18/04/2017)
Gideon's first ability will continue to prevent damage dealt by the target permanent even if Gideon leaves the battlefield before your next turn.
(18/04/2017)
If Gideon leaves the battlefield after giving you his emblem, you'll keep the emblem. Its effect will apply again later if another Gideon comes under your control.
