Those are always edge cases. I would be hesitant to provide a simple rule on how to deal with these kinds of answers, although one thing should be clear: "You cannot do it" can be a valid answer.
However in this particular instance it's clearly this:
It doesn't solve the real problem. If OP asks "Is possible", he really means "How do I do ".
In general, it doesn't make a lot of sense to ask whether something is possible. Asking how to achieve it is always preferred, and if the answer is, "it's not possible", then so be it. Clearly, in this case, the OP needs to do something very specific – a workflow in GIMP – and just telling them you can't do it is
- not particularly helpful
- maybe not even true (who knows, maybe it's possible but the answerer hasn't found a way to)
So, this question here can't be compared with others where you can actually prove that something cannot be done. Unless you can technically argue that it's not possible – and will never be – then that's a full answer to the question. I could come up with plenty of examples where this would be acceptable: Imagine asking how you can write data to a CD-ROM, or how you can make a low quality compressed movie high quality. It's just not feasible.