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Gideon's Defeat says:
Exile target white creature that's attacking or blocking. If it was a Gideon planeswalker, you gain 5 life.
However the rules also say:
Planeswalkers are not creatures. Spells and abilities that affect creatures won't affect them.
If planeswalkers are not creatures, you cannot exile them with Gideon's Defeat, and thus you are not gaining any life from it?
Planeswalkers usually aren't creatures, but planeswalkers can sometimes turn into creatures. All iterations of Gideon can turn into a creature—it's kinda his thing—and sometimes other planeswalkers can too, like Sarkhan, the Dragonspeaker who can turn into a dragon.
In Amonkhet, the Gideon planeswalker was Gideon of the Trials. His second loyalty ability says:
Until end of turn, Gideon of the Trials 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.
After using this ability his type line would say: Legendary Planeswalker Creature — Gideon Human Soldier. This makes him a legal target for "target creature", so if he then attacks, he'll be a legal target for Gideon's Defeat.
Since you usually can't activate loyalty abilities on someone else's turn, it's rare you'll see Gideon blocking like this, but if somehow that happens he'll be a legal target for Gideon's Defeat then too.
Planeswalkers are not automatically creatures, but there are effects that can make them become creatures.
For example, Gideon Jura has an ability that says:
Until end of turn, Gideon Jura becomes a 6/6 Human Soldier creature that's still a planeswalker. Prevent all damage that would be dealt to him this turn.
After this ability is used, if Gideon Jura attacks or blocks, he would be a valid target for Gideon's Defeat.
I believe this to be because -as the others have stated-, some Planeswalkers have the ability to become creatures. Specifically, all Gideon Planeswalker Cards have this ability. This would be the basis of why this card was created: To target specifically Gideon to give you a boost of Life.
This is the same for any of the other Planeswalker defeat cards in Hour of Devastation. They all have specific requirements that relate to certain perks that each of the named Planeswalkers have.
In magic, planeswalkers are not considered creatures, this is true. But they can be made creatures by other effects. Most specifically the Gideon planeswalkers all have an ability that "animates" them, makes them into creatures, from the original Gideon Jura with his ability:
0: Until end of turn, Gideon Jura becomes a 6/6 Human Soldier creature that's still a planeswalker. Prevent all damage that would be dealt to him this turn.
To the most recent, Gideon of the Trials with his ability:
0: Until end of turn, Gideon of the Trials 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.
While these are active, Gideon can attack like any other creature, following the normal creature rules, untapped and not summoning sick etc. The abilities only last for the turn, so they don't usually allow Gideon to block, unless there is a way to activate them on opponents turns, like through the emblem of Teferi, Temporal Archmage.
There are other ways to animate planeswalkers, usually that involves making them some other card type that can be easily made a creature. The common way to do this is with a card called Mycosynth Lattice, which makes everything, planeswalkers included, into an artifact, and there are many cards that can turn artifacts into creatures, like March of the Machines or Animate Artifact, even some planewsalkers themselves, like Tezzeret, Agent of Bolas could turn themselves into a creature.
