Gideon Jura

If you activate Gideon Jura's third ability and then unpreventable damage is dealt to him (due to Unstable Footing, for example), that damage has all applicable results: specifically, the damage is marked on Gideon Jura (since he's a creature) and that damage causes that many loyalty counters to be removed from him (since he's a planeswalker). If the total amount of damage marked on Gideon Jura is lethal damage, he's destroyed as a state-based action. If Gideon Jura has no loyalty counters on him, he's put into his owner's graveyard as a state-based action.

If a creature controlled by the affected player can't attack Gideon Jura (because he's no longer on the battlefield, for example), that player may have it attack you, another one of your planeswalkers, or nothing at all.

Gideon Jura's first ability applies during each combat phase of the affected player's next turn (as opposed to applying during the affected player's next combat phase). The distinction is relevant if there are no combat phases during that turn (due to Fatespinner's effect, for example) or there are multiples (due to World at War, for example).