Setup OIDC with Terraform.gitlab-ci.yml migration into OpenTofu

Question

I'm currently in the process of transitioning from using Terraform for managing my GitLab CI/CD pipelines to using OpenTofu. In this migration, I also need to integrate OIDC (OpenID Connect) authentication into my GitLab pipelines.

Previously, my .gitlab-ci.yml file looked something like this with Terraform:

You should upgrade to the latest version. You can find the latest version at https://gitlab.com/gitlab-com/gl-security/security-operations/infrastructure-security-public/oidc-modules/-/releases
include:
  - remote: 'https://gitlab.com/gitlab-com/gl-security/security-operations/infrastructure-security-public/oidc-modules/-/raw/3.1.2/templates/gcp_auth.yaml'
  - template: "Terraform/Base.gitlab-ci.yml"

variables:
  WI_POOL_PROVIDER: //iam.googleapis.com/projects/$GCP_PROJECT_NUMBER/locations/global/workloadIdentityPools/$WORKLOAD_IDENTITY_POOL/providers/$WORKLOAD_IDENTITY_POOL_PROVIDER
  SERVICE_ACCOUNT: $SERVICE_ACCOUNT
  TF_ROOT: infrastructure
  TF_STATE_NAME:tfstate

stages:
  - validate
  - test
  - build
  - deploy
     
validate:
  extends: .terraform:validate
  needs: []

build:
  extends:
    - .google-oidc:auth
    - .terraform:build
deploy:
  extends:
    - .google-oidc:auth
    - .terraform:deploy
  dependencies:
    - build

Now, I want to replace the Terraform-based setup with OpenTofu, while also incorporating OIDC authentication into my pipeline. However, I'm unsure about how to structure the .gitlab-ci.yml file and configure OpenTofu to achieve this.

Could someone provide guidance on how to migrate from Terraform to OpenTofu for GitLab CI/CD pipelines, particularly focusing on integrating OIDC authentication into the pipeline setup? Any examples, tips, or resources would be greatly appreciated. Thank you!