Gideon Blackblade's first ability reads as a Characteristic-Defining Ability (604.3 & 604.3a) to me, but the 'as long as its your turn' clause is causing some uncertainty.
Specifically, does the timing restriction fall foul of the "it is not an ability that an object grants itself" clause?
This question can generally be summed up as "Can I cast Gideon Blackblade on my turn using Aluren or will Cavern of Souls naming Human make him uncounterable?"
Gideon's first ability is not a characteristic-defining ability because it "set(s) the values of" Gideon's power, toughness, and subtypes (here, "4/4 Human Soldier" [C.R. 205.3m]) "only if certain conditions are met" (in this case, only "[a]s long as it's your turn") (C.R. 604.3a). Compare Gideon Blackblade with Angry Mob.
Because Gideon's first ability is not a characteristic-defining ability (and isn't otherwise an ability mentioned in the subrules of C.R. 113.6), it works only while Gideon is on the battlefield (C.R. 113.6; compare with C.R. 113.6a), so that, for example, that ability won't make Gideon a creature spell on the stack for the purposes of Aluren (C.R. 108.1) or Cavern of Souls.
EDIT (Aug. 8, 2020): Some rules were renumbered in the meantime.
Gideon Blockblade is a Planeswalker Spell on the stack , at that time you cast it , it 's just a Planeswalker spell. The 1st Static ability of Gideon only function on the field , which means in the other Zones (except on the field) it's a Planeswalker.
EDIT. Because the 1st static ability of Gideon Blockblade is not Characteristic-defining ability.
604.3a A static ability is a characteristic-defining ability if it meets the following criteria: (1) It defines an object’s colors, subtypes, power, or toughness; (2) it is printed on the card it affects, it was granted to the token it affects by the effect that created the token, or it was acquired by the object it affects as the result of a copy effect or text-changing effect; (3) it does not directly affect the characteristics of any other objects; (4) it is not an ability that an object grants to itself; and (5) it does not set the values of such characteristics only if certain conditions are met.
Specifically, does the timing restriction fall foul of the "it is not an ability that an object grants itself" clause?
This question can generally be summed up as "Can I cast Gideon Blackblade on my turn using Aluren or will Cavern of Souls naming Human make him uncounterable?"
A Dying Wish
To Rise Again
Chainer, Dementia Master
Muldrotha, the Gravetide
Atraxa, Praetors' Voice
Because Gideon's first ability is not a characteristic-defining ability (and isn't otherwise an ability mentioned in the subrules of C.R. 113.6), it works only while Gideon is on the battlefield (C.R. 113.6; compare with C.R. 113.6a), so that, for example, that ability won't make Gideon a creature spell on the stack for the purposes of Aluren (C.R. 108.1) or Cavern of Souls.
EDIT (Aug. 8, 2020): Some rules were renumbered in the meantime.
EDIT. Because the 1st static ability of Gideon Blockblade is not Characteristic-defining ability.
604.3a A static ability is a characteristic-defining ability if it meets the following criteria: (1) It defines an object’s colors, subtypes, power, or toughness; (2) it is printed on the card it affects, it was granted to the token it affects by the effect that created the token, or it was acquired by the object it affects as the result of a copy effect or text-changing effect; (3) it does not directly affect the characteristics of any other objects; (4) it is not an ability that an object grants to itself; and (5) it does not set the values of such characteristics only if certain conditions are met.
So Aluren or Cavern of Souls have no effect with Gideon.
A Dying Wish
To Rise Again
Chainer, Dementia Master
Muldrotha, the Gravetide
Atraxa, Praetors' Voice