These days Gideon is more of a creature than a planeswalker, but it doesn't make him less valuable. On the contrary, a 4/4 creature with indestructible for three mana is something every creature-based deck wants to play.
His minus ability is definitely exciting, but a bit expensive. This will surely set many players aback, as historically Gideon's ultimate abilities have almost always been very cheap to activate.
As a result, Gideon Blackblade could drop in price, and the limit of it depends how effectively it will be used in current top-tier decks.