Greg quit doing drugs a long time ago. As far as I know Hep C is a blood transmitted disease and can not be caught from drinking booze. It was not reported that he had cirossis, though Hep C makes one more susceptable to other forms of liver damage.
Gregg Allman claimed in a radio interview he contracted Hepititis C from a tatoo parlor.
Cirossis is a disqualifying condition for eligibility to be placed on a donor list, AFAIK.
I have heard varying stories with respect to how he contracted the HepC. In his case, I would guess injectable drugs, but there's probably no way to know for sure, so they don't generally disqualify for that condition.