The 'GCP Firewall Rule Deleted' detection rule identifies instances where firewall rules in Google Cloud Platform (GCP) have been deleted. Such actions may lead to unintended service disruptions if the deletion is not deliberate. This rule triggers alerts based on GCP Audit Logs that detail the operations performed on firewall rules. The alerts are designed to notify administrators about deletions initiated through various methods including 'compute.firewalls.delete' and 'appengine.Firewall.DeleteIngressRule'. The severity of the alerts is categorized as low, which indicates that while such deletions should be monitored, they may not pose immediate threats unless they coincide with other unusual activities. A runbook associated with the rule advises verification to confirm that deletions are expected, to prevent potential operational impacts. Additionally, the testing framework is set up to ensure alerts trigger appropriately for delete actions while avoiding false positives for non-delete actions related to firewall management. A reference link is provided for further reading on GCP firewall management.