That does seem like a public health concern. Normally you'd recover damages from a class action lawsuit or similar, but it sounds like it was common practice at the time so nobody was acting negligent. That's just a sad state of affairs.
Gilead - or maybe even Congress/local states in cooperation with Gilead - could get a lot of goodwill by offering the cure for free in a one-time deal to people who:
* Developed Hepatitis C before 1992.
* Had a blood transfusion that likely contributed to the illness
* Can't afford to pay for the cure otherwise. To start with, let's say that you make less than $60k/year to qualify.
* Don't have insurance that would cover it
It seems about as justifiable as when the government steps in to help after a natural disaster.