Creating the GRETAP interface
Following the method used on site A's firewall, create the GRETAP interface:
Name: assign a name to the GRETAP interface
Bridge: select an existing bridge on the firewall. This may be a bridge created by the default configuration or a bridge created specifically for this purpose.
GRETAP tunnel configuration
Tunnel source: select the physical interface through which GRE traffic will travel on the firewall. In the example given, this is the Firewall_out interface.
Tunnel destination: select an object bearing the public IP address of the remote firewall (Remote_Firewall in the example).
Click on Finish then Apply to confirm the creation of the interface.