pfSense

Normally removing an IP from an interface where that type is "none" is appropriate. But not with gif (or tun or tap or GRE or any other interface type that's addressed elsewhere).

To replicate, you can follow our tunnel documentation (no need for an actual functional tunnel, can just check ifconfig on fake one if necessary).
https://doc.pfsense.org/index.php/Using_IPv6_on_2.1_with_a_Tunnel_Broker

after setting up the gif, you have:

: ifconfig gif0
gif0: flags=b051<UP,POINTOPOINT,RUNNING,LINK0,LINK1,MULTICAST> metric 0 mtu 1280
    tunnel inet 10.29.9.150 --> 10.15.25.14
    inet6 2001:470:fff0:54c::2 --> 2001:470:fff0:54c::1 prefixlen 128 
    inet6 fe80::20c:29ff:fe6b:34a%gif0 prefixlen 64 scopeid 0x7 
    nd6 options=21<PERFORMNUD,AUTO_LINKLOCAL>

After assigning it (step "Configure OPT Interface" in above tunnel doc) and applying changes, you have a broken tunnel:

: ifconfig gif0
gif0: flags=b051<UP,POINTOPOINT,RUNNING,LINK0,LINK1,MULTICAST> metric 0 mtu 1280
    tunnel inet 10.29.9.150 --> 10.15.25.14
    inet6 fe80::20c:29ff:fe6b:34a%gif0 prefixlen 64 scopeid 0x7 
    nd6 options=21<PERFORMNUD,AUTO_LINKLOCAL>