Windows Server instances use password authentication instead of SSH authentication. 
To prevent unauthorized access to new Windows instances, Compute Engine requires that you generate a new Windows password for that instance before you connect to it. 
In the GCP Console:
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Go to the VM instances page.
 
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Click the Windows instance where you want to generate a new password.
 
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On the instance details page, click the Set Windows password button.
 
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The username field specifies which Windows user receives the new generated password. If you want to create a new user, specify a new username in this field and Compute Engine will add that user with a new password.
 
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Click Set.