To allow Compute Engine instances to access public repositories for security updates while an egress firewall rule is in place to deny all internet traffic, you need to create a more specific egress rule that permits traffic to the CIDR range of the repository. The priority of this rule should be lower (i.e., a higher priority number) than the deny rule.

Steps:

Identify the CIDR Range: Determine the CIDR range of the public repository from which the security updates will be fetched.

Create Egress Firewall Rule: Create a new egress firewall rule allowing traffic to the identified CIDR range with a priority less than 1000.

Apply Firewall Rule: Use the Google Cloud Console or gcloud command-line tool to apply the new firewall rule.