To create a repository you can simply add the lines corresponding and GitRoot will create it.
Open .gitroot/repositories.yml in the root repository:
1root:
2 defaultbranch: main
3 owners: []
4+ repo1:
5+ defaultbranch: main
6+ owners:
7+ - your_pub_ssh
8+ - ssh-ed25519 AAA...
By default GitRoot use main has defaultBranch but you can put every name you like.
Commit your change and push git add . && git commit -m "create repo1 repository" && git push. When the change will be in the defaultBranch GitRoot will create it.