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:
Go to the VM instances page.
Click the Windows instance where you want to generate a new password.
On the instance details page, click the Set Windows password button.
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.
For creating a Custom password refer to : https://cloud.google.com/compute/docs/instances/windows/creating-passwords-for-windows-instances