Gustcloak Savior has
Whenever a creature you control becomes blocked, you may untap that creature and remove it from combat.
What about creatures with Vigilance? Attacking does not cause them to tap so they cannot be untapped. Can they still be removed from combat, after they become blocked?
Yes, you are allowed to use Gustcloak Savior's ability to remove an attacking creature from combat, even if it is not tapped.
Gustcloak Savior's ability is a triggered ability, that triggers in your Declare Blockers step for each of your attacking creatures that have become blocked. Then, when the ability resolves, it lets you choose whether to have the ability's effect happen. Rule 608.2d says in part
If an effect of a spell or ability offers any choices other than choices already made as part of casting the spell, activating the ability, or otherwise putting the spell or ability on the stack, the player announces these while applying the effect. The player can’t choose an option that’s illegal or impossible
Rule 609.3 says
If an effect attempts to do something impossible, it does only as much as possible.
Since part of the effect can happen (removing the creature from combat) and it will just not do the impossible part (untapping the creature), it's not considered impossible, so you are allowed to choose it.
Then, once you choose to apply the effect, it tries to do both things: untap the creature and remove the creature from combat. Since the creature is already untapped, it is impossible to untap it, so it only does the part that is possible, which is removing that creature from combat.
This ruling has been confirmed by the official Ask A Magic Judge blog, which says
"Untap that creature and remove it from combat" is the action you’re taking, so as long as you perform some part of that action, you can choose to perform the action and remove the creature from combat.
Great question! This is a confusing thing for less experienced players. We know from 701.20b that (emphasis mine)
To untap a permanent, rotate it back to the upright position from a sideways position. Only tapped permanents can be untapped.
This would seem to indicate that we're doing something illegal. However, there is a difference between an effect and a cost. Even though only tapped permanents can be untapped, you can still attempt to untap an already untapped permanent. I tried very hard to find a rule that supported this claim and thanks to the incredibly common terms "tap" and "untap" I simply couldn't find it. But I did find a supporting ruling on a card.
Deadshot is a sorcery that says
Tap target creature. It deals damage equal to its power to another target creature.
And the supporting ruling states:
Deadshot can target a creature which is already tapped as its first target, and it will still damage the second target. This is because it taps that creature as an effect, not as a cost.
So clearly this is saying that an already tapped card can be targeted with intent to tap it and the ability won't fizzle. A lot of the cards do not have that ruling on them, but it clearly still applies. It must just not be a common enough problem to justify a ruling put on it anymore.
