Cisco 827 protocol forwarding

Hi,

I'm trying to confiigure a cisco 827 to foward requests from the internet
back to an internal server. I've got them working for UDP and TCP, but i was
wondering how I go about setting them up for other protocols, such as 47
(gre). Computers connect via NAT to the outside world.
Internally I have a PPTP VPN server which needs to be externally accessible.
I've got the control line (tcp port 1723) forwarded ok, but I just need the
data channel now.....
Here is the config line for my control channel:

ip nat inside source static tcp 172.20.0.1 1723 interface Dialer1 1723


when i try the obvious:

ip nat inside source static 47 172.20.0.1 interface Dialer1

it crashes out saying that the '47' is invalid. is there a way to do this
properly??


Thanks
Mike