Suppose murder just is wrongful killing. Then it seems that Sally ought not murder Bob follows from no premises, the empty set of premises. Trivially, the empty set of premises is a set containing only descriptive premises, in Hume’s sense. But then, Sally ought not murder Bob, a substantive normative claim, follows from a set of purely descriptive sentences. So, you can derive an ought from iss. Take that, is-ought gap.
I have some half-baked potential responses in mind, but let’s see what you think.