To be very honest, I think gift points used should not be dynamically calculated and should be calculated similar to how points available is done instead. If, say, I give a user 30 days of donor usergroup membership and the donor user earns that member an additional 5 gift points per day, we run into a problem when the member's usergroup subscription expires; I assume they lose all those extra gift points.

The field is not designed as some kind of increment/decrement on top of another calculation, it was designed to actually hold the absolute value of the points used. So if you say they've used "-1000" then that's it, end of story.

We no longer rely on it for positive values for the reason you said, it is better to dynamically calculate it.

And setting a negative value in there has very questionable use, it's basically a hack in the code that users would never normally find, for a very rare use case.

There'd be an argument for removing the field entirely, but I suppose it's just there for consistency, as the various other fields exist and some of them are strictly needed while others help with performance.