Any suggestions from experience?
You can check the docs for the GCP provider to see if the resources you want to manage are "importable" into the Terraform state file; they usually are and you'll see a section at the bottom of each resources documentation page showing you how to do this. e.g. https://registry.terraform.io/providers/hashicorp/google/lat...
Your process will be -
1. Write TF configuration approximating what you think is deployed
2. Import all your resources into the state file
3. Run a `terraform plan ...` to show what Terraform wants to change about your resources (including creating any you missed or changing/recreating any your config doesn't match)
4. Correct your TF configuration to reflect the differences from 3.
5. Goto 3, repeat until you get a "No changes" plan or the you actually want TF to correct some things (add tags, for example)
6. run `terraform apply`
and optionally...
7. set up your CI/automation to run `terraform plan` regularly and report the "drift" via some means - stuff that has been changed about your resources outside of Terraform management.
I put a lot of stock in this last step, because small, incremental change is the cornerstone of platform management. If you want to make a change and come to find there's a huge amount of other stuff you have to correct as well, your change isn't small any more.
Use `import` resources in a .tf file (I like to just call it imports.tf) and run `terraform plan -generate-config-out=imported.tf`
That will dump the tf resources - often requires a little adjustment to the generated script, but it's a huge time saver