.git/.graphite_repo_configof your repositiory. On first execution (per repository), or by directly running
gt repo init, the CLI will prompt you to provide a minimal amount of info to populate this config (the trunk branch for your development flow).
repo initoffers the option to add existing branches to Graphite by stacking them onto your trunk branch. Many users will not need this and just use Graphite for new branches — if that is the case, feel free to skip.