Nullification is based on a contradiction in terms: juries cannot be punished for the judgments they render, and prosecutors may not re-try a case to obtain their preferred outcome (“double jeopardy”).

With a grand jury, the second term doesn’t apply: the grand jury’s decision doesn’t prevent a second indictment.

So in other words: grand juries can choose to render “faithless” decisions, but they can’t nullify a case in the sense that matters.