I worked on rpm-ostree 2018.5,and added gnome packages in centos-atomic-host.json, also modified default_target: "graphical.target". After I composeed tree, and rpm-ostree rebased to the new repo, the atomic host still run in multi-user.target. But, the atomic host will work run in graphical.target after "systemctl set-default graphical.target".
I want to know what's wrong with my procedure, thank you.