The way the terminus beasts dissolve into an unpleasant ooze when they're slain in the benchmark trailer as opposed to simply dissipating into aether like the Lunar primals does seem to suggest that they may be of dissimilar origin.

If the "gateway of the gods" is some kind of teleportation device meant primarily for providing easy access to the moon, it could just as easily be used to transport hordes of biological monstrosities created by the Allagan tech the Garleans are known to have hijacked (Which might be what we were seeing at Thavnair in the Shadowbringers trailer).