OpenBSD VPN Question

I am new to OpenBSD. I did a quick search of the archives and didn't find anything recent/relevant on this question. So forgive me if I missed it somewhere. I am redesigning my company firewall from a Dual-Homed Architecture to a Screened Subnet Architecture. I currently run Linux as the host and I have about 20 machines on the internal network using PPTP clients to connect to a PPTP server a few states away. I want to implement a Linux machine as our Exterior router and OpenBSD as our Interior router. I know the Linux machine will pass/filter GRE packets, with a patch, but I'm not sure if OpenBSD will. Can I use ipf to allow/deny GRE? If not are there any patches/ports that will allow me to do this?