Enchanted creature loses all abilities and has base power and toughness 1/1.
Relevant bit of Gideon's text:
As long as it's your turn, Gideon Blackblade is a 4/4 Human Soldier creature with indestructible that's still a planeswalker.
Gideon stays a vanilla 1/1 creature since he was a creature at the time
Gideon loses the ability that makes him a creature, immediately stops being a creature and drops the aura
Gideon stays a 1/1 until opponent's turn ends, then stops being a creature and drops the aura