NAT and GRE Packet Forwarding - Cisco 2500 router

[KiinG]

I am trying to access a Windows PPTP VPN through a Cisco router that is doing NAT. I am doing port forwarding for the TCP port 1723 and it work fine. My problem is forwarding GRE packet. When I do "ip nat inside source static ?" on the router I only have choice between TCP and UDP. I was wondering if there was a way - other that replacing the router - to forward GRE packet. The router is an old Cisco 2500.

Any help would be appreciated,

Benjamin