There are no facts I know of why Gavin Andresen's creditability should take any hit so far.
The fact that Gavin has said he believes this guy to be Satoshi when his false signature claim was debunked within hours shows that Gavin has lost all credibility (where is Gavin's response to the debunking?).
The evidence that Gavin (allegedly) saw was not the debunked signature. Gavin has claimed to have seen a signed message from the address that received the coinbase transaction for the block subsidy from block "1".
If this can be proven to be true then I would say that Wright is reasonably satoshi.
If it was true (that he could produce a valid signature) then why would you go to the effort of posting such a hoax on your website?
It has been speculated that Wright was providing journalists a way/method to verify a signed message (believe it or not, very few people have the technical understanding of encryption to understand what a signed message really means).
And why doesn't Gavin just post the public key and signature?
(hint - he can't)
It has been said that Gavin was not able to keep the signed message.
You should perhaps be considering Occam's razor here (the simplest explanation being that Craig Wright does not have the private keys he is claiming to have).
That is possible, maybe even probable. I am not currently convinced that Wright is satoshi, however I don't think it is possible to disprove this either. I am willing to keep an open mind and consider additional evidence as it comes out.