I've read conflicting rulings on this matter, perhaps someone can clear things up.
I read a ruling that when a Planeswalker becomes an INDESTRUCTIBLE creature, (ie. several different Gideons), damage can still deplete loyalty and as a result be sent to the graveyard. This doesn't make sense to me. What point would there be in making it an indestructible creature if combat can still kill it? (You can use symantecs and say it's not actually being destroyed, but that exact process would still be happening.
You're just applying different terminology, which is absurd.)
Any loyalty counters on Sarkhan when his first ability resolves will remain on him while he’s a creature. Because he’s not a planeswalker at this time, damage dealt to him won’t cause those counters to be removed.
Shouldn't this bolded bit be applicable to all planeswalkers?
It is true that "a planeswalker [that] has loyalty 0 [is] put into its owner's graveyard" (rather than being destroyed) as a state-based action (C.R. 704.5i), so that indestructible will not affect that event (review C.R. 702.12b). But this is also the case even if the planeswalker is also a creature, has indestructible, and has lethal damage (so that even if C.R. 704.5g-h would destroy the planeswalker, but don't due to indestructible, C.R. 704.5i can still bring that planeswalker to the graveyard if it has loyalty 0). Here, loyalty acts like toughness, rather than damage marked on a creature (compare C.R. 704.5f with C.R. 704.5g-h). (Why this is the case is not a rule question.)
Although destroying a permanent involves "mov[ing] it from the battlefield to its owner's graveyard" (C.R. 701.7a), the reverse is not true (see, e.g., C.R. 701.15, sacrifice).
Under C.R. 119.3c, "[d]amage dealt to a planeswalker causes that many loyalty counters to be removed from that planeswalker". That rule doesn't apply to a nonplaneswalker permanent, even if that permanent is normally a planeswalker.
It is worth pointing out that you also missed an important part of Gideon's ability: the part where damage to him is prevented. This is why his own ability functions intuitively but something like Heroic Intervention would not. Since Heroic Intervention does not prevent damage, making a planeswalker indestructible in this manner will often accomplish very little against creatures dealing damage to it.
Also, as peteroupc points out, the part you bolded doesn't apply to *any* planeswalkers. The reason for that ruling is because it applies when Sarkhan is no longer a planeswalker so there is nothing causing Loyalty counters to be removed.
Take a close look at Sarkhan's ability: ''...Sarkhan, the Dragonspeaker becomes a legendary 4/4 red Dragon creature with flying, indestructible, and haste.''
Now compare with Gideon's ability: ''...Gideon Jura becomes a 6/6 Human Soldier creature that's still a planeswalker...''
Gideon becomes a creature AND a planeswalker: it could be killed by some planeswalker-bane (but preventing damage makes it much more difficult).
Sarkhan becomes a creature only; it entirely ceases to be a planeswalker, for a while. Damage does not remove Loyalty counters from Sarkhan-dragon... because that only happens to planeswalkers!
Rule 205.1b: Some effects change an object’s card type [...] but specify that the object retains a prior card type [...]. This rule applies to effects [...] that state that something is “still a [type, supertype, or subtype].” [...]
Rule 205.1a: Some effects set an object’s card type. In such cases, the new card type(s) replaces any existing card types. [...]
I read a ruling that when a Planeswalker becomes an INDESTRUCTIBLE creature, (ie. several different Gideons), damage can still deplete loyalty and as a result be sent to the graveyard. This doesn't make sense to me. What point would there be in making it an indestructible creature if combat can still kill it? (You can use symantecs and say it's not actually being destroyed, but that exact process would still be happening.
You're just applying different terminology, which is absurd.)
Furthermore, I present an opposing ruling from the official site:
http://gatherer.wizards.com/Pages/Card/Details.aspx?multiverseid=386650
Shouldn't this bolded bit be applicable to all planeswalkers?
Although destroying a permanent involves "mov[ing] it from the battlefield to its owner's graveyard" (C.R. 701.7a), the reverse is not true (see, e.g., C.R. 701.15, sacrifice).
Under C.R. 119.3c, "[d]amage dealt to a planeswalker causes that many loyalty counters to be removed from that planeswalker". That rule doesn't apply to a nonplaneswalker permanent, even if that permanent is normally a planeswalker.
Also, as peteroupc points out, the part you bolded doesn't apply to *any* planeswalkers. The reason for that ruling is because it applies when Sarkhan is no longer a planeswalker so there is nothing causing Loyalty counters to be removed.
''...Sarkhan, the Dragonspeaker becomes a legendary 4/4 red Dragon creature with flying, indestructible, and haste.''
Now compare with Gideon's ability:
''...Gideon Jura becomes a 6/6 Human Soldier creature that's still a planeswalker...''
Gideon becomes a creature AND a planeswalker: it could be killed by some planeswalker-bane (but preventing damage makes it much more difficult).
Sarkhan becomes a creature only; it entirely ceases to be a planeswalker, for a while. Damage does not remove Loyalty counters from Sarkhan-dragon... because that only happens to planeswalkers!
Rule 205.1b: Some effects change an object’s card type [...] but specify that the object retains a prior card type [...]. This rule applies to effects [...] that state that something is “still a [type, supertype, or subtype].” [...]
Rule 205.1a: Some effects set an object’s card type. In such cases, the new card type(s) replaces any existing card types. [...]
RULES OF MAGIC :
http://magic.wizards.com/en/game-info/gameplay/rules-and-formats/rules