According to Harry Potter and the Deathly Hallows, Gregorovitch was once the owner of the Elder Wand. Grindelwald stole the Elder Wand from Gregorovitch without defeating him in a duel. So it would seem Grindelwald actually never became the real owner of the Elder Wand, therefore Dumbledore, who defeated Grindelwald in a duel, would also have never become the real owner, which means neither Draco nor Harry would have ever mastered the Elder Wand.
Wasn't Gregorovitch killed by Voldemort? Wouldn't that make Voldemort the real master of the Elder Wand, and therefore able to kill Harry?