How does Gamora know the location of the
Did I miss something or is this never explained? How does Gamora know the trial needed to acquire it?
I don't have sources other than my own understanding of the movie, but it's explained that:
Gamora was sent on a mission by Thanos many years ago to find the Soul Stone. It's not explained where she acquired this information, just that she had, and that she was lying about not knowing where it was.
As for the trial:
I don't believe she knew what the trial was until she and Thanos were told by the stone's keeper. My basis for this is her reaction; she goes from somber to mirth when she thinks there's no way for Thanos to complete the trial.
I'm just going to expand what Paul D. Waite said in the comments to the question. From the movie:
While interrogating / torturing Nebula, Thanos replays her memories, particularly the one where Gamora tells her that she found the map to the Soul Stone and burned it to ashes.
But Thanos obviously doesn't believe Gamora, saying something along the lines of:
He brought her up and never trained her to lie, and that's why she (Gamora) is so bad at it. So Thanos is guessing that Gamora either didn't burn the map, or burned the map after committing to memory where the Soul Stone is.