Objectively it's morally wrong to gain from someone's loss. So... winning anything? Schadenfreude? A profitable short position? Picking a penny up from the ground?

Anyway, the specifics aside... how do you arrive at the conclusion that it is objectively wrong to gain from someone else's loss?

You're correct on your judgment but your opinion stems from a different social stance on the underlying issue.

(that sounded really pedantic and it was not my intention)

Morality is a human creation. By default, nothing is wrong or right, until a human mind, be it an individual or colective one, analyse it and evaluate it.

This does not mean you can not view something as being immoral while others do or understand it as not a moral concern. This difference of understandingb is what moves any subject into the moral/immoral spectrum.

I think your confusing objective with universal. They aren't the same thing. You can have an objective truth that isn't universal. Let's use Euclidean and non-Euclidian geometry as an example, both have very objective truths, but they aren't the same or interchangeable.

The problem with saying slavery is universally immoral is that you label most everyone that existed prior to the modern era as immoral, because slavery was foundational across the globe in multiple cultures. Morality isn't useful if you can actually compare or contrast because even involved was immoral.