Gideon of the Trials — ruling 1

Gideon's first ability will continue to prevent damage dealt by the target permanent even if Gideon leaves the battlefield before your next turn.

Gideon of the Trials — ruling 2

Gideon's second ability doesn't count as a creature entering the battlefield. Gideon was already on the battlefield; he only changed his types.

Gideon of the Trials — ruling 3

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).

Gideon of the Trials — ruling 4

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).

Gideon of the Trials — ruling 5

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.

Gideon of the Trials — ruling 6

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.

Gideon of the Trials — ruling 7

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.

Gideon of the Trials — ruling 8

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.

Gideon of the Trials : règles, fonctionnement et interactions (MTG)

Gideon of the Trials is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

+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."

How it works : Gideon of the Trials

Original English text — Wizards of the Coast / Scryfall

#1April 18, 2017· Wizards of the Coast
Gideon's first ability will continue to prevent damage dealt by the target permanent even if Gideon leaves the battlefield before your next turn.
No other card is named in this ruling.
#2April 18, 2017· Wizards of the Coast
Gideon's second ability doesn't count as a creature entering the battlefield. Gideon was already on the battlefield; he only changed his types.
No other card is named in this ruling.
#3April 18, 2017· Wizards of the Coast
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).
No other card is named in this ruling.
#4April 18, 2017· Wizards of the Coast
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).
No other card is named in this ruling.
#5April 18, 2017· Wizards of the Coast
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.
No other card is named in this ruling.
#6April 18, 2017· Wizards of the Coast
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.
No other card is named in this ruling.
#7April 18, 2017· Wizards of the Coast
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.
No other card is named in this ruling.
#8April 18, 2017· Wizards of the Coast
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.
No other card is named in this ruling.
#9April 18, 2017· Wizards of the Coast
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.
No other card is named in this ruling.