You cannot configure GRE VPN between controller and APs on the same local switch.
VPNs are used to tunnel traffic between remote/different subnets, and require a
VPN termination (that uses GRE). Typically you have your controller in a Data Center
and APs are at remote sites. Can you have a read through the SZ Tunneling Interface
reference guide, and see if this answers some of your questions?
https://support.ruckuswireless.com/documents/1188-smartzone-3-4-1-mr-tunneling-interface-reference-g...