He's not human, so who can say what his people's naming conventions are?
It's unclear whether he's human or not. Garth is a fairly ordinary human name of Norse or possibly Welsh origin. He's "of Izar," but Izar could be a human colony (which is how the novels interpret it), or it could just be a place he's associated with for reasons other than his birth, like Lawrence of Arabia (who was English) or "Zefram Cochrane of Alpha Centauri."
But since Garth is repeatedly addressed as "Captain Garth," that tells us that Garth is treated as his surname for the purposes of English speakers and official records. That's why it logically follows that his placement on an alphabetical list would be based on that name instead of some other.