The big problem is that as soon as you defeat Zeal in the Black Omen, you initiate the first stage of the fight with Lavos, and Gaspar's dialogue switches to the lines about the final battle, whether the other side quests are completed or not. Thus the quest can't be checked against Gaspar's comments the way all the others are, and for that reason people are driven nuts by the idea that there might be a sidequest they're missing out on.