This should still be at least a wishlist to change the man page to clarify that -g only works for -L port forwarding. In the SSH docs, "port forwarding" is pretty much described the same for -L and -R forwarding, so the -g description in the manual should make clear that it is only for -L forwarding. Either that, or it should do as it says in the manual and allow remote hosts to connect to the forwarded port during a -R forwarded port.