Since Google Compute Engine does not allow root user nor assign any password to the default Owner Account.
I though the SSH console in the Compute Engine backend can SSH to the instance regardless the SSH Config.
Obviously I was wrong, I modified sshd_config file and did not put the default owner account in the allowUsers parameter. Right now, I cannot SSH to the instance using owner account thus lost any SUDOER right and was stuck.
I however have set up a normal user which has no SUDOER rights but can SSH to the instance.
Is there any way to solve this or I have to rebuild the server?