About the elder wand?

Whoa. I had to reread that, it was confusing. But I think that since Grindelwald stole it directly from Gregorovitch, that makes it his, just as Harry stealing Draco's wand directly made it his. He didn't have to kill Draco. I know that it was rumored that the wand had to be passed on by murder, but I think that was just a rumor.

Did that make any sense? Sorry, it was a bit unclear. I think Voldemort killed Gregorovitch, so even though Grindelwald wasn't master, he had to die to make Dumbledore master, who had to die. But that made Draco master, because he stole it from Dumbledore by disarming him, just like with Grindelwald. Voldemort mistakenly thought Snape stole the wand, so he killed him, but Snape was never master.

Meanwhile, Harry gets Draco's wand. This makes him master of the wand. I think. Which is why Harry said he was master at the end.

Harry (and Ollivander) both state that a wand also has to pick its wizard. The elder wand selected Draco as its new "master" when Draco magicked it away from Dumbledore when he cornered the headmaster on the astronomy tower in book 6. Draco had no idea what the wand was and left it behind, which explains why it was buried with Dumbledore. (Read the last full paragraph of page 742 of "Deathly Hallows" again. so much is happening in this scene that you might have missed it.) Hope this helps!

Grindewald didn't need to kill him, just take it by force. Grindewald did fire a curse off at Gregorovitch, so i guess that counts as beating him in a very brief duel.

maybe grindewald and gregorovitch engaged in a dual and grindewald beat him but gregorovitch vanished with the wand so gridnewald had to steal it from him

Gregorovitch said he never owned it and I think he meant that, I think it was true. Grindelwald must have defeated its real master before he took it from Gregorovitch, I am thinking Gregorovitch knew he wasn't the wand's master and that is why he told Voldemrt it was never his.