It looks like hostname.if(5) in OpenBSD has had it's syntax slightly changed. Here's how I've configured my end-point gif tunnel in OpenBSD 4.7.
$ cat /etc/hostname.gif0
tunnel 174.136.x.y 66.220.y.x
inet6 2001:470:X:X::2 128
dest 2001:470:Y:Y::1
!route -n add -inet6 default 2001:470:Y:Y::1
Using "prefixlen" seems to cause problems in the current release, so apparently the above is the best way to configure your tunnel.