Since Dumbledore's and Grindelwald's duel occurred in continental Europe, where Gregorovitch resided, did Gregorovitch ever find out that Grindelwald wielded it during his legendary 3-hour duel, and later lost mastery of the Elder Wand to Dumbledore?
When interrogated by Voldemort, (who is also clearly reading Gregorovitch's mind), Gregorovitch doesn't know who it was that stole the wand from him [Gregorovitch]:
'Do not lie to Lord Voldemort, Gergorovitch. He knows....he always knows'.
'Who was the thief, Gregorovitch? said the high cold voice'
'I do not know, I never knew, a young man - no - please, PLEASE!'
- Harry Potter and the Deathly Hallows - Ch. 14 The Thief
Under interrogation by Voldemort, Gregorovitch made no mention or even suggest that it was Grindelwald who stole the wand, and since it seems clear that Voldemort killed Gregorovitch when the interview was over, it is unlikely he made the connection in the few short seconds he had remaining to him
Grindelwald however, did apparently lie to Voldemort about possessing the wand
At last he [Harry] said, "Grindelwald tried to stop Voldemort going after the wand. He lied, you know, pretended he never had it."
- Harry Potter and the Deathly Hallows - Ch. 35 King's Cross
So it is possible that Gregorovitch also lied to Voldemort, though less likely given that Voldemort told Gregorovitch not to lie (the implication being he knew Gregorovitch was obfuscating initially). The next statement Gregorovitch makes appears to be more of the whole truth than his original statement. Grindelwald was welcoming death, knew he was going to die, and didn't care (Ch 23) , whereas Gregorovitch was pleading for life. Additionally, we can assume that Gregorovitch is similar to Ollivander, in that he is a talented wand maker but not a "powerful" wizard like Voldemort or Grindelwald (in his prime) and thus would be extremely fearful of Lord Voldemort and not wish to resist him.