The following input errors were detected:
The gateway address xxxx:xxxx:xxxx:ba8::1 does not lie within one of the chosen interface's subnets.

Problem is that is indeed the interface subnet (This is on a GIF tunnel with HE.net).
Interface IPv6 subnet: xxxx:xxxx:xxxx:ba8::2/64.

However in the actual underlying config, I see this:
On the CLI login welcome screen:

 HENETV6_US (opt6) -> gif0       -> v6: xxxx:xxxx:xxxx:ba8::2/128

and ifconfig gif0:

gif0: flags=8051 <up,pointopoint,running,multicast>metric 0 mtu 1280
        options=80000 <linkstate>tunnel inet xx.xx.xx.xx --> 209.51.161.14
        inet6 xxxx:xxxx:xxxx:ba8::2 --> xxxx:xxxx:xxxx:ba8::1 prefixlen 128 
        inet6 fe80::20c:29ff:feba:cb9e%gif0 prefixlen 64 scopeid 0x11 
        nd6 options=21 <performnud,auto_linklocal></performnud,auto_linklocal></linkstate></up,pointopoint,running,multicast> 

Notice the /128 netmask?!  If the web gui is checking the actual interface netmask as opposed to the config entered into the Interface screen, then I can understand why this is failing.

Anyone experience this?