Since the bird is not very agile I would run away until I could find some smaller animals to kill with my battleaxe. Then I would offer them to the Gastornis, thereby gaining its trust. We would pace the prehistoric highways together, slightly awkwardly at first because of the species difference, but then one day a big angry mammal would leap out at me and raise its claw to tear my throat out, when the Gastornis charges at it and headbutts it away from me, stunning it and allowing me to get some battleaxe hits in. From that moment on we would realise that we are kindred spirits, and the bird would even let me ride it around a bit, although not too much because that would be Quite Weird.
It's me, a battleaxe and a 6-foot extinct predator, and it'd be a feel-good buddy comedy.