In English law Gary would probably be alright - As a general rule stolen goods are recoverable by the original owner as sales of chattels (like guitars) are about the transfer of title to the goods. The claim is not in theft (which is branch of criminal law) but in conversion and trespass to goods.
The bottom line is that a thief has no title to pass (he having never acquired a legitimate right to the goods) and therefore no can one derive 'title' from him to sell the goods. The consequence is that the original owner retains title and so can call for the goods to be returned. If you buy a stolen car or guitar, title doesn't pass to you, although you have a claim for your money back from the seller (if you can find him, that is!)
However, if the purchaser (or a chain of purchasers before him) purchased the goods in good faith (meaning they had no knowledge that the goods purchased were stolen) and has had possession of the goods for in excess of 6 years (or can show a chain of good faith transactions for in excess of 6 years) without a claim or notification from the original owner, then the original owner is barred by ss.2-4 of the Limitation Act 1980 from recovering the goods and his title is extinguished.
That's the law of England, but I have no idea about the law of California - As to American judgments - the English Courts may sometimes enforce them - but it is very complicated law. The fact that Gary Moore lives in the UK, the remedies against him in California are punitive and that he has not submitted to the jurisdiction of the US court tends to mean that the UK won't allow the enforcement. It may mean that Gary Moore won't be able to go to California though!
(sorry this is probably a bit heavy for a guitar board - and its all from memory, so it could contain errors)
