Gideon of the Trials

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

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.

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.

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

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.