Creating a new App Engine application in GCP can be done using the Google Cloud Console, CLI commands, or Terraform. Here, we will explain each approach in detail:

1. Google Cloud Console:
   a. Go to the GCP Console: https://console.cloud.google.com/
   b. Select your project or create a new one.
   c. Navigate to the App Engine dashboard by clicking the hamburger menu, then click on “App Engine.”
   d. Click “Create Application.”
   e. Select a region for your application and click “Next.”
   f. Choose a runtime environment (e.g., Python, Java, Node.js, Go, etc.) and click “Next.”
   g. Follow the instructions to create the app.yaml file and deploy your application.
2. CLI Commands (gcloud):
   a. Install and set up the Google Cloud SDK: https://cloud.google.com/sdk/docs/install
   b. Authenticate with your GCP account: gcloud auth login
   c. Set the active project: gcloud config set project PROJECT_ID
   d. Create an app.yaml file with the necessary configurations (runtime, service, etc.).
   e. Deploy the App Engine application: gcloud app deploy app.yaml
3. Terraform:
   a. Install Terraform: https://learn.hashicorp.com/tutorials/terraform/install-cli
   b. Create a main.tf file with the required provider and resource configurations:

provider "google" {
  project = "PROJECT_ID"
  region  = "REGION"
}

resource "google_app_engine_application" "app" {
  project     = "PROJECT_ID"
  location_id = "REGION"

  serving_status = "SERVING"

  feature_settings {
    split_health_checks = true
  }
}

resource "google_app_engine_standard_app_version" "app_version" {
  version_id = "v1"
  service    = "default"
  runtime    = "python39"

  entrypoint {
    shell = "gunicorn -b :$PORT main:app"
  }

  deployment {
    zip {
      source_url = "https://storage.googleapis.com/YOUR_BUCKET/YOUR_APP_SOURCE.zip"
    }
  }

  env_variables = {
    VAR_NAME = "VAR_VALUE"
  }

  automatic_scaling {
    cool_down_period_seconds      = 120
    max_instances                 = 5
    min_instances                 = 1
    target_cpu_utilization        = 0.75
    target_throughput_utilization = 0.75
  }
}

resource "google_storage_bucket" "bucket" {
  name = "YOUR_BUCKET"
}

c. Initialize the Terraform project: terraform init
d. Review the Terraform plan: terraform plan
e. Apply the Terraform plan: terraform apply

Replace the placeholders (e.g., PROJECT_ID, REGION, YOUR_BUCKET, and YOUR_APP_SOURCE) with the appropriate values for your project.