The quick answer is VCD does not support IPv6.
If you want to use IPv6, set the gateway to a valid IPv4 gateway (the edge device will only use IPv4) and then once the vApp is online with IPv4, go to each VM and add IPv6 entries as you require. Your VM's (if using VXLAN) will be able to talk via IPv6 (however the gateway out of the network and connections in will need to be IPv4). If you want to do routing of IPv6, you an use VXLAN or even VLAN backed network Pools.
Virtual machines managed by vCloud Director using IPv6 can only communicate to endpoints that are not behind vCloud Network and Security Edge devices. Edge does not currently support IPv6. Virtual machines that communicate on the same directly attached vApp or organization virtual datacenter network can use IPv6. To communicate with the outside world using IPv6, connect the organization’s virtual machines to a direct external organization virtual datacenter network.
IPv6 service on Canopy TAI is unavailable and not supported on both Canopy TAI versions (1.2 & 2.0)