Create a scheduled job in Cloud Run to invoke a container every minute. The container will check the application URL. If the application is down, switch the URL to the 'Site is unavailable' page, and notify the Ops team.

Create a cron job on a Compute Engine VM that runs every minute. The cron job invokes a Python program to check the application URL. If the application is down, switch the URL to the 'Site is unavailable' page, and notify the Ops team.

Create a Cloud Monitoring uptime check to validate the application URL. If it fails, put a message in a Pub/Sub queue that triggers a Cloud Function to switch the URL to the 'Site is unavailable' page, and notify the Ops team.

Use Cloud Error Reporting to check the application URL. If the application is down, switch the URL to the 'Site is unavailable' page, and notify the Ops team.