If planeswalkers aren't creatures, how does Gideon's defeat work?
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&#39;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.