Gaiasia was a freshwater predator which was exceptional among stem-tetrapods for its combination of relatively enormous size, Southern occurrence, and late survival.

Though limb material is not preserved, the skull of Gaiasia indicates that its affinities lie with digit-bearing stem-tetrapods (early amphibians, in the broad sense).

Other digit-bearing stem tetrapods were significantly smaller (skulls under 40 centimetres (16 in) in length), and nearly all were restricted to the tropics of Euramerica, a low-latitude region equivalent to present-day Europe and North America.

Gaiasia hints that stem-tetrapods in Southern latitudes continued to persist and evolve through the Late Paleozoic icehouse interval, even as low-latitude species died out and were supplanted by crown-tetrapods.

[1][2] Gaiasia has a broad flattened skull, large keeled branchial elements (throat or gill bones), a reinforced neck, and massive interlocking fangs at the front of the snout.

Paired exoccipital condyles have convergently evolved in a number of crown-tetrapod lineages (lissamphibians, stereospondyls, some "lepospondyls", and mammals), none of which are related to Gaiasia.

On the edge of the braincase, a spur of bone divides the spiracular cleft (a hole which hosted the spiracle or ear canal) from the depressor mandibulae fossa (a pocket for the muscles which open the mouth).