Its fossils were discovered in several formations dating to 73.5 and 71.5 million years ago in what is now northern Spain, Southern France, and Romania.

It coexisted with large predators like abelisaurid theropods, herbivores such as ankylosaurians and titanosaurian sauropods, as well as pterosaurs, crocodylomorphs, turtles, fish, and various archaic birds.

Southern France and north-western Spain where its fossils are found was part of the large Ibero-Armorican island in the prehistoric Tethys Sea.

The type locality of Gargantuavis, the Bellevue site in the Marnes Rouges Inferieures Formation, is 71.5 million years old (earliest Maastrichtian).

[8] Although its fossils are rare, the presence of Gargantuavis from southeastern France to north-western Spain shows that this bird had a wide distribution in the Ibero-Armorican island.

[12] It is possible that Gargantuavis lived mainly in an environment that was not compatible with fossilization, such as areas far from the rivers and floodplains, which represent most of the fossiliferous deposits in the late Campanian-early Maastrichtian of France and Spain.

The titanosaur Ampelosaurus, found together with Gargantuavis in the Bellevue site, shows also a reduction in its growth rate, possibly linked to some environmental pressure like periodic food shortages.

This is supported by sedimentological and mineralogical studies which indicate episodes of semi-arid and strongly seasonal climate during the Late Cretaceous in Southern France.

The archaic Hațeg avian theropods Elopteryx and Balaur bear some similarity to Gargantuavis remains, which may indicate the three form some clade native to the Late Cretaceous European archipelago, though they have ambiguous affinities.