I am new to FreeBSD but have used BSD since 20 years back…
In the man-page for rc.conf I can see…
(bool) The IPv6 equivalent of gateway_enable.
(bool) If set to "YES", configure host to act as an IP
router, e.g. to forward packets between interfaces.
If this is equal to set net.inet6.ip6.forwarding=1 in sysctl.conf it would for clarification be a good thing to mention in the man-page for rc.conf that this is an alternate way of setting this sysctl setting. A simple small parentesis containing "Equal to set net.inet6.ip6.forwarding=1 in sysctl.conf" would be good.
AFAIR, they are not equal. devd can reapply network configuration on some events, and if you have not *gateway_enable option in rc.conf, you will lost your gateway, because sysctl.conf does not applied in the same time.
They are not equivalent, gateway_enable="YES" is a superset of net.inet.ip.forwarding=1