Chapter 11 IPsec VPN for FortiOS 5.0 : GRE over IPsec (Cisco VPN) : Configuring the FortiGate unit : Configuring the GRE tunnel : Adding GRE tunnel end addresses
  
Adding GRE tunnel end addresses
You will also need to add tunnel end addresses. The Cisco router configuration requires an address for its end of the GRE tunnel. Using the CLI, enter tunnel end addresses that are not used elsewhere on the FortiGate unit, like this:
config system interface
edit gre1
set ip 10.0.1.1 255.255.255.255
set remote-ip 10.0.1.2
end