I already had Gungnir, and I sold it, since I didn't know it was going to be a quest item later and none of my characters use spear.

Okay, so question 1: is this an important quest? Should I be worried if I can't finish it?

Question 2: Is there something like the Sage was in MM6 that will return quest items you've accidentally sold?

Question 3: If I return to where I found it, will that black chest again have Gungnir or some other artifact? (And yes, I've forgotten where I found it, but don't spoil it for me, I'm willing to re-do all the dungeons if necessary.) About how long until that dungeon resets?