https://www.warhammer-community.com/2020/09/08/lumineth-and-friends/
A largely empty article with only one valuable piece of information, in my opinion.
"Just who is Bundo Whalebiter? Allow us to explain! Sons of Behemat is coming out before too long, and in it, you won’t just find rules for an army of gargants, but a set of mercenaries that let you add a named Mega-Gargant to your armies! Order and Destruction forces can take Bundo Whalebiter, a towering (and surprisingly cunning) Kraken-eater. If your Lumineth Realm-lords need some extra punch, make sure to grab him when the Mega-Gargant kit drops!"
Sounds to me like Sons of Behemat are going to have rules for various named Gargants that can be taken for different factions. I expect there will be mercenaries for Death and Chaos as well... but, I'm a little doubtful on the Death honestly. Solid chance they won't be available, but who can really say?