Cannot ping default gateway. The firewall seems to not build dynamically an ARP entry for the IP of its default gateway.
Resolution
Review your NAT policies.
The firewall performs proxy ARP on the IPs configured for inbound NAT as destinations. A network object that includes the IP address of the default gateway is commonly used in the destination field of a NAT rule. This will result in the proxy ARP process preventing the firewall from building an ARP entry for the firewall's default gateway.