Originally Posted by
Cilia
I'd like to say it's pre- and post-fall Lahabrea on the top and bottom, respectively, but the bottom man appears to be wearing the standard Amaurotine mask and not Lahabrea's unique Convocation mask (unlike the top one who at least appears to be holding said Lahabrea mask).
The other two are obviously Erichthonios and Themis (a.k.a. Elidibus), while there isn't enough detail on the figure in the doorway to figure anything out.
The Lahabrea - Pandaemonium connection is easily drawn with the derivative esper from Ivalice - Mateus, the Corrupt is the Scion of Darkness to Lahabrea, Scion of Light in Ivalician mythos, with Mateus' name coming from Final Fantasy II's Emperor Mateus Palamecia, who after dying and taking over Hell, returned to Earth as the Emperor of Darkness and raised Pandaemonium from the depths as his palace.
I doubt it has anything to do with the GF from VIII; that was a one-off summon, and a rather unimpressive and immemorable one at that. (And yes, I know other one-off summons have been used before, but those were all big end-game ones; Knights of the Round, Eden, and Ark.)