The present - or just past - Vlaakith has been queen of her race for "only" 1000 years. How long has Zaerith been around?
Since the beginning of the githzerai race?
Countless millennia?
2000 years?
1000 years?
A few centuries?
A decade?
Or maybe nobody has any idea.
Were there other kings before Zaerith? Were they also named Zaerith?
Zaerith Menyar Ag-Gith doesn't seem to be as fundamental to his people as Vlaakith is to hers. It's possible the githzerai don't really have a tradition of kings, and Zaerith is a relatively new innovation.
Zaerith is "apparently" immortal. Why is this?
Is he an outsider?
A quasi-deity?
A prolonger?
An incantifer?
An Athasian-style dragon king?
Undead?
Does he simply reincarnate, generation after generation?
Is he not immortal at all?
I read "apparently immortal" as "has been around longer than this chronicler or the people he talked to". Which could mean something conclusive, but probably doesn't.
I would say the 'zerai do have a tradition of kings, though they don't take it as seriously as the 'yanki.
IMO, Zaerith Menyawhatever isn't really such an important figure at all. There really is no githzerai society as such to rule over - the anarchs are self-employed, the rrakkmas too, the monasteries ignore him - what else is there? The 'zerai commoners that pay him lip service? They might call him "god-king", but I would say the 'zerai interpretation of the word "god" is akin to the japanese "kami" - a powerful and mysterious "spirit" that deserves respect (and not blind worship).
I like the idea of prior kings of the same name. Maybe it's the status itself, the symbolic position that the githzerai respect (because it signifies Unity), and not the man who holds it.