How to get password for sudo

Per https://cloud.google.com/compute/docs/instances ,

The instance creator and any users that were added using the metadata sshKeys value are automatically administrators to the account, with the ability to run sudo without requiring a password.

So you don't need that non-existent password -- you need to be "added using the metadata sshKeys value"! The canonic way to do that, and I quote from that same page:

$ echo user1:$(cat ~/.ssh/key1.pub) > /tmp/a
$ echo user2:$(cat ~/.ssh/key2.pub) >> /tmp/a
$ gcloud compute project-info add-metadata --metadata-from-file sshKeys=/tmp/a

or you can use the Google Developers Console for similar purposes, see https://cloud.google.com/compute/docs/console#sshkeys if you'd prefer that.

To become another non-root user on a GCE VM, first become root via password-less sudo (since that's how sudo is configured on GCE VM images):

sudo su -

and then switch to the user you want to become or run a command as another use, e.g., in your case, that's:

sudo -i -u postgres