A confirmed gap is a sub-query where retrieval returns results — the question is being answered somewhere on the web — but the brand has no content that answers it. The gap is confirmed because the sub-query is real and being served; the brand is simply absent from the answer.
Confirmed gaps are the highest-priority content opportunities because the demand is proven. A retrieval system found results; the brand just was not among them. The fix is building content that answers that specific sub-query better than the current incumbent. The competitive baseline is known — search results show who is currently filling the gap.