To import existing resources into Terraform, you need to do two things1:
Write a resource configuration block for each resource, matching the type and name used in your state file.
Run terraform import for each resource, specifying its address and ID. There is no such command as terraform Import-gcp, and provisioning new VMs with the same names will not import them into Terraform.

A secure string is not a valid option to keep secrets out of Terraform configuration files. A secure string is a feature of AWS Systems Manager Parameter Store that allows you to store sensitive data encrypted with a KMS key. However, Terraform does not support secure strings natively and requires a custom data source to retrieve them. The other options are valid ways to keep secrets out of Terraform configuration files. A Terraform provider can expose secrets as data sources that can be referenced in the configuration. Environment variables can be used to set values for input variables that contain secrets. A -var flag can be used to pass values for input variables that contain secrets from the command line or a file.
References = [AWS Systems Manager Parameter Store], [Terraform AWS Provider Issue #55], [Terraform Providers], [Terraform Input Variables]

This is the meta-argument that you must include in any non-default provider configurations, as it allows you to give a friendly name to the configuration and reference it in other parts of your code. The other options are either invalid or irrelevant for this purpose.

This is the variable type that you could use for this input, as it can store multiple attributes of different types within a single value. The other options are either invalid or incorrect for this use case.

The best way to automatically and proactively enforce the security control that new AWS S3 buckets must be private and encrypted at rest is with a Sentinel policy, which runs before every apply. Sentinel is a policy as code framework that allows you to define and enforce logic-based policies for your infrastructure. Terraform Cloud supports Sentinel policies for all paid tiers, and can run them before any terraform plan or terraform apply operation. You can write aSentinel policy that checks the configuration of the S3 buckets and ensures that they have the proper settings for privacy and encryption, and then assign the policy to your Terraform Cloud organization or workspace. This way, Terraform Cloud will prevent any changes that violate the policy from being applied. References = [Sentinel Policy Framework], [Manage Policies in Terraform Cloud], [Write and Test Sentinel Policies for Terraform]