This is pretty interesting, because it's one of those things where the more meta we are or aren't as voters, the more the situation changes. On the most fundamental level: Gratitude is promising something specific, and the soul oath has to mean that it's something she's serious about. She believes she can and will be able to fulfill it, or else she believes Lōkahi will release her from the oath. Lōkahi knows this, so they know she has to mean it.
We also know from just... the last two updates... that no matter what else is or isn't true, Gratitude possesses treasures and potential resources that greatly surpass the Salted Spires' current powers, so Lōkahi has a good reason to believe that she can make good her promise, even without context we have.
What Lōkahi can't know and which readers of the canon books do is that... her story does seem to check out. She looks like an Akura, she's named like an Akura, her path appears to be in line with common Akura paths, and she name-dropped their Sage, Charity... which not only suggests 'Akura', but 'fairly high-up in the Akura head family'. Charity is her 'Aunt', a position Charity holds for a lot of Akura as an honorific, even when not literally true. Charity isn't that old, so it can't be too much before canon. This almost certainly isn't post-canon, because the reference to "my great-grandmother" as a champion against dragons only fits Akura Malice's and Seshethkunaaz the Dragon King's relation up through partway into canon.
There's still potential for her to be lying, which is to say that this could be a trick on the readers (presumably Watsonianly because she's doing an Akura impression on a third party for some reason), but I don't think she is.
The thing with Akura virtue names is that... people live up to them, even those who don't appear to at first. Mercy is easy. Pride is easy. Grace is easy. Malice has explicit in-story encyclopedia entries explaining her. But then you have the generally-chill Fury, which doesn't make sense until you see him cut loose against someone he actually doesn't like. Charity seems standoffish and cold, but she genuinely does care for and give to the family and clan with selfless charity. Even Harmony wants a type of harmony, just a self-centered harmony of unquestioned hierarchy.
Lōkahi thus doesn't know, but we do, that Gratitude is probably going to show real gratitude, perhaps in a slightly warped way.
We can make other more or less meta analyses: what if this is supposed to be the start of a protagonist cheat skill? if the question is up for a vote in a KittyEmpress quest, there has to be up and downsides both ways, right? what will other scavengers among the Spires think of this deal? how far will said scavengers want to push things to get at Gratitude's valuables? if she was betrayed, is someone with the 'betrayers' likely to show up and maybe offer us a comparable/better deal with different context?
I think it's definitely worth considering her offer and maybe even taking it up, but my personal reaction here is basically that I picture Lōkahi reacting more based on what he knows and expects and what he thinks his family will default to, as opposed to an impulsive or dispassionately rational take. Ironically, both of those could see Lōkahi accepting Gratitude's offer, but I'm instead going to vote for:
[x] B. "No, I will not make that oath. I won't serve you."
Wouldn't it be fascinating to meet back up with Mom and Tama and have them say "what the hell why did you say no"?
