GRE tunnels are configured under Interfaces > (assign) on the GRE tab.
GRE (Generic Route Encapsulation) is defined in RFC 2784. It is supported by many vendors, and is a standards-compliant means to tunnel traffic.
GRE tunnels can be managed from the list view.
Parent interface - Interface on which to bind the GRE tunnel (typically a WAN)
Remote tunnel endpoint IP address - Far side peer address outside the tunnel
Local tunnel IP address - Local address for inside the GRE tunnel (typically a small private network)
Remote tunnel IP address - Remote address for inside the GRE tunnel (typically a small private network)
The following are optional:
Mobile encapsulation: Check this box to use mobile encapsulation (IP protocol 55, RFC 2004). When unchecked, uses GRE encapsulation (IP protocol 47, RFC 1701, RFC 1702).
Route search type: For correct operation, the GRE device needs a route to the destination that is less specific than the one over the tunnel. (Basically, there needs to be a route to the decapsulating host that does not run over the tunnel, as this would be a loop.
WCCP version: Check this box for WCCP encapsulation version 2, or leave unchecked for version 1.
Description - Text for reference about the name/purpose of this tunnel.
Traffic must be passed on the GRE interface and there is no general tab for GRE traffic. This means that it is required to add and then enable it under Interfaces > (assign). When doing so, keep the IP types set to None on the Interface page as that is managed by the configuration here on the GRE tab.