If you have a Snapgear/ Cyberguard unit as your internet facing firewall you can still forward pptp packets into an internal pptp server by adding the following custom rules  in the firewall section of the unit.

Ofcourse you cant have a pptp server running on the snapgear itself at the same time. In the example below the pptp server is running on an internal server on 192.168.0.1

iptables -t nat -A PREROUTING -i $INTERNET_IF -d $INTERNET_ADDR -p 47 -j DNAT --to-destination 192.168.0.1
iptables -A ExtAcc -p 47 -d 192.168.0.1 -j ACCEPT