再起動後、SSHを介してGoogle Cloudサーバーにアクセスできなくなります。

最善の方法は、gcloudコンソールを使用して手動でキーを作成することです。

thufir@mordor:~$ 
thufir@mordor:~$ gcloud compute config-ssh
WARNING: The private SSH key file for Google Compute Engine does not exist.
WARNING: You do not have an SSH key for Google Compute Engine.
WARNING: [/usr/bin/ssh-keygen] will be executed to generate a key.
Generating public/private rsa key pair.
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in /home/thufir/.ssh/google_compute_engine.
Your public key has been saved in /home/thufir/.ssh/google_compute_engine.pub.
The key fingerprint is:
SHA256:ioreurewiopruweoiprwe thufir@mordor
The key's randomart image is:
+---[RSA 2048]----+
.......
|                 |
|                 |
+----[SHA256]-----+

Updated [https://www.googleapis.com/compute/v1/projects/<project>].
You should now be able to use ssh/scp with your instances.
For example, try running:

  $ ssh <instance>.<location>.<project>

thufir@mordor:~$ 
thufir@mordor:~$ ssh <instance>.<location>.<project>
The authenticity of host 'compute.<####> (<ip address>)' can't be established.
ECDSA key fingerprint is SHA256:<fingerprint?>.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added 'compute.<####>' (ECDSA) to the list of known hosts.
Welcome to Ubuntu 16.04.1 LTS (GNU/Linux 4.4.0-31-generic x86_64)

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$ 
$ 
$ whoami
thufir
$ pwd
/home/thufir
$ exit
Connection to <ip address> closed.
thufir@mordor:~$ 

源泉：

https://askubuntu.com/questions/803908/

また見なさい:

JSONを使用してSSHキーをGoogle Cloud VPSに接続するには？