These are the only bosses in the castle you can draw GFs from:

Tri-point: Siren
Krysta: Carbuncle
Droma: Leviathan
Red Giant: Pandemona
Gargantua: Cerberus
Catoblepas: Alexander
Tiamat: Eden

So Sphinaxur is the only boss (exempting Ultimecia) that has no GF to draw.