pfSense

The following field (and maybe others) seem not to be properly validated:

GIF tunnel local address
GIF tunnel remote address

I entered a "/64" at the end of both of these fields, and it appeared to save successfully, but then failed at this step:

php-fpm 12554 /system.php: The command '/sbin/route change -host -inet6 2001:xxx:xx::2 2001:xxx:xxxx:xxx::1/64' returned exit code '71', the output was 'route: 2001:xxx:xxxx:xxx::1/64: hostname nor servname provided, or not known'

Fixing my mistake - removing /64 from those fields, disabling and re-enabling the gateway - fixed the problem; but it should have been caught sooner, at the GIF page.