Gaston Plante, known for his exploits in the realm of physics, actually discovered the fossils of Gastornis, named after him clearly, as a young academic but is forgotten as a paleontologist due to his success in physics. That is quite all right. His name adorns a genus comprised of four species with three proposed genera now viewed as synonyms (Diatryma, Barornis and Omorhamphus). All three synonyms were reappraised as species of Diatryma and then as one solid species of Gastornis. The species belonging to the genus are G. parisiensis (discovered by Plante near Paris, France), G. giganteus (originally Diatryma from Cope 1876), G. sarasini (Schaub 1929), and G. russeli (Martin 1992). Gastornis was a forest dweller and the only Terror Bird known in Europe. Terror Birds were typically from South America until the area between Mexico and Venezuela became a solid land bridge as it is today which allowed the migration of Terror Birds from south to north (and saber tooth cats from north to south). How Diatryma and Gastornis came to be considered one in the same is how Gastornis came to be recognized as one of the Terror Birds of the Americas. Terror Bird is a general label of large birds during the Cenozoic era; it only truly applies to the South American birds with Gastornis being a single European/North American genus and the large Australian birds of the time being referred to as Mihirungs. However, the bigger question is how did it evolve on a completely separate continent in the dense forests of Europe to be considerably like its large plains dwelling cousins in the Americas, even if it is thought to be related to the duck?
An idea, and this is by no means and official out of a scholarly paper idea, is that their ancestors, also being related to ducks, may have had some flight muscles and may have also been long distance floaters. Since the Americas and Europe were once rather close together, perhaps it is possible that the Diatryma branch and the Gastornis branch, prior to both becoming enormous angry avians, could have dispersed amongst the two continents in this manner. However, it is also feasible that, due to the archipelago like nature of Europe during the Paleocene and Eocene, the closeness of Greenland, and the closeness of North America to Greenland, that these birds could have simply island hopped after becoming, already, the large apex predators of their areas. Regardless of how they moved, the anatomy of Gastornis would have required a specific land type for success as a predator.
Gastornis would not have been able to survive in open plains. This is supported by the legs (pun slightly intended) of Gastornis. The legs of this bird were not built for speed over distance but rather sudden short bursts of power, as we would find in successful ambush predators. This would be perfect for the dense forest and the pudgy little prey we may find there like Leptictidium and Hyracotherium (Eohippus). Conversely, the ambush predator theory has been dropped by some who believe that scavenging (there is always someone with large predators that feels this way) was the main mode of nutritive intake for Gastornis. Since Gastornis is sometimes thought to have been a ratite it is considered by some to have been an herbivore or to a point and omnivore by some as well, eating small insects, mammals, and lizards as the opportunity presented itself as well as plant matter. The beak, though large and small-mammal-crunch-able, could have been used to crush large seeds easily, which supports the herbivorous idea. The large beak and the strong legs support scavenging (rending and tearing decomposing flesh) and the ambush predator (short burst of speed with a giant beak to smash or crush prey) ideas. More study of the skeleton is needed to clearly pinpoint a diet, of course a last meal would also do a great deal of help toward this purpose also.