I tried to remove all the gnome-* snaps in /var/lib/snapd/seed/seed.yaml and run "snap abort --last=seed", "sudo systemctl restart snapd".
$ sudo snap install gnome-3-28-1804
It prompt download the core18(20190508 970).
So this hang is related to the wrong version of ubuntu core.
I tried to remove all the gnome-* snaps in /var/lib/ snapd/seed/ seed.yaml and run "snap abort --last=seed", "sudo systemctl restart snapd".
$ sudo snap install gnome-3-28-1804
It prompt download the core18(20190508 970).
So this hang is related to the wrong version of ubuntu core.