pfSense

A gif tunnel (with tunnelbroker.net) has been configured with /64 nets in GIF settings, as well as on OPT6 interface - probably because it is a point-to-point connection. Adding a (default) gateway in interface.php works, editing the route in system_gateways.php fails the input validation stating: "The gateway address 2001:470:1f0a:22b::1 does not lie within one of the chosen interface's subnets." which is plausible when you look at status_interfaces.php

Workaround: When gateway has to be edited, it is possible to delete the gateway and re-add it from the interface as new gateway.

HENETV6 Interface (opt6, gif0)
Status up
MAC Address 00:00:00:00:00:00 - Xerox
IPv6 Link Local fe80::21b:21ff:fe2c:ba6d%gif0
IPv6 Address 2001:470:1f0a:22b::2
Subnet mask IPv6 128
MTU 1480
In/out packets 389789/308690 (187.18 MiB/19.13 MiB)
In/out packets (pass) 389789/308690 (187.18 MiB/19.13 MiB)
In/out packets (block) 24/0 (2 KiB/0 B)
In/out errors 0/0
Collisions 0

also in routing table:
2001:470:1f0a:22b::1 link#19 UH 694 1280 gif0
2001:470:1f0a:22b::2 link#19 UHS 0 16384 lo0