DCGW: TF how to for GCP VPC

## Jobs to be done (optional)

## Definition of done
- TF how to for GCP VPC https://github.com/Kong/terraform-provider-konnect/blob/main/examples/scenarios/cloud-gateways.tf
- There is a DCGW TF Skill you should use. Invoke with `/dcgw-terraform-how-to` in Claude. If you need to make any changes or it does anything incorrectly, please have it update the Skill files.

## Information
https://app.kapa.ai/93b55f65-cffb-4d0b-9da7-c8915a838dce/top-questions/clusters/1d751e26-2daa-4bee-9236-982724a556be

## Due date (optional)

## Size
L

Start by reviewing examples/scenarios/cloud-gateways.tf in the terraform-provider-konnect repository and the linked Kapa information. Use the /dcgw-terraform-how-to Skill in Claude, updating its files if needed. Done means adding a Terraform how-to for a GCP VPC and confirming it covers the requested DCGW scenario.

Written by the indexing model from the issue text.