Say you activate Gideon Jura's third ability, then an opponent gains control of him before combat. You may have any of your creatures attack Gideon Jura (since he's still a planeswalker). Then Gideon Jura may block (since he's a creature). He may block any eligible attacking creature, including one that's attacking him! During combat, he behaves as an attacked planeswalker and/or a blocking creature, as appropriate. For example, he deals combat damage to any creatures he's blocking, but he doesn't deal combat damage to any unblocked creatures that are attacking him.