"Whenever Gustcloak Sentinel becomes blocked, you may untap it and remove it from combat".
It's a 3/3. Say a 3/3 creature is declared to block it. Does either creature die? In other words, is damage dealt by Gustcloak Sentinel before it is removed from combat?
By the time Gustcloak Sentinel's ability resolves, it will still be the declare blockers step, so well before creatures assign, let alone deal, combat damage. If Gustcloak Sentinel is removed from combat, it will no longer be an attacking creature, so that—
any creatures that blocked Gustcloak Sentinel (such as the blocking creature in your scenario) will remain in combat but won't get to assign combat damage to Gustcloak Sentinel, and
Gustcloak Sentinel won't get to assign combat damage to those creatures.
Can the blocking creature then be re-assigned to block another attacking creature? I would assume no, or else I don't see the point of Gustcloak Sentinel's ability.
Can the blocking creature then be re-assigned to block another attacking creature? I would assume no, or else I don't see the point of Gustcloak Sentinel's ability.
Gustcloak Sentinel's ability doesn't make any creatures block any other creatures. (See False Orders and Balduvian Warlord for examples of effects that do.)
It's a 3/3. Say a 3/3 creature is declared to block it. Does either creature die? In other words, is damage dealt by Gustcloak Sentinel before it is removed from combat?
Can the blocking creature then be re-assigned to block another attacking creature? I would assume no, or else I don't see the point of Gustcloak Sentinel's ability.
RULES OF MAGIC :
http://magic.wizards.com/en/game-info/gameplay/rules-and-formats/rules