>Gruber is right. His specific answer comes from his correct knowledge. I know he is correct because he's specific.
Erm, that's a very odd way of putting it. I think the OP was just saying "Gruber's specific claims have been reliable in the past, and hence are quite likely to be correct now." That is perfectly reasonable and it's not circular. (It is, of course, an inductive form of appeal to authority, but there is nothing wrong with that.)
As I first read it, I thought "he's right because he's specific and he's specific because he's right", which would be circular; but while the he's all refer to Gruber, the rights are distinct.
He thinks he is right therefore he is specific, because he is specific I think he is right.
That's what I was trying to say in my awkward earlier post. That does seem appealing to authority - thinking Gruber is right because of what he says and how he says it, rather than considering why he says it.
Actually, what riled me more was the interpretation
"He thinks he is right therefore he is specific, because he is specific I think he thinks he is right."
Which conveys absolutely no information and adds nothing to the discussion, just repeating that G thinks he is right, which is a given, and was still getting upvotes; so I tried to attack the post to feel better, banking on pedanticism being good for a few karma points around here anyway.
It might be an appeal to authority, but there isn't any circularity. If you're going to play fallacy bingo, at least get it right...