What to do with gaps in mathematical papers? Assume the following is given in a published paper:


There is an important result.
The important result might contain a gap, as possibly some people agree.
The authors never respond to questions on that and so maybe they do not want to make it public that there is a gap, which can not be filled.
It is not likely to give a direct counterexample to the important result, but only to point out possible gaps.


What to do then? The problem is obviously that some people might quote and use the result in new papers. How about the idea that such gaps could be marked in the published version of the paper as a motivation for future research? Of course a recent editor of the journal should choose a new referee to decide whether to mark such a paper.
Also I think this should not have bad consequences for the authors as there might be reasons one is not aware of.
If the paper has not been published yet and the gap is brought to the attention of the editor and/or referee, they should inform the authors of the problem and ask for it to be corrected. If the authors don't correct the gap then the paper should be rejected.

If the paper has already been published, anyone else who finds the gap, brought it to the attention of the authors and didn't receive a satisfactory reply has two main formal options that I can think of:


Write to the journal which published the paper. The journal can decide what to do, with an obvious step being to retract the paper. For an important result that was published in a serious journal, I expect the journal to take the matter quite seriously and solicit opinions from experts (which is essentially the same as the "gap referee" idea in your question) about whether such a drastic measure should be taken. In the event of a decision to retract, the journal will issue a statement that it no longer endorses the correctness of the paper, and probably add an explanation of the reasoning behind the decision.
Alternatively, the discoverer of the gap can write and publish a paper pointing out the gap.


In both of these approaches, the existence of the gap has entered the official scientific record and most other researchers in the field will become aware of it, so the risk of people erroneously relying on an unsubstantiated result is reduced, though not completely eliminated. The risk can be further reduced by publicizing the gap through other informal channels such as blogs, seminar and conference talks, emails to groups of colleagues, posts on MathOverflow, etc.