this is a protocol number 47, NOT PORT 47 !!!
As you already noticed, this goes along with tcp or udp or icmp, other VPN connections use ESP (protocol no. 50) and/or AH (proto 51).
Have a look at the /etc/protocols file for a short definition. If using GRE-protocol, you are using L2TP, then ?
Just as a side note SuSEfirewall doesn't support pptp traffic. It can be compiled as a beta version into IPTABLES. So if thats what your trying dont waste your breath. If you feel brave go get the latest pom patch from netfilter.org and install the pptp masquerading patch. Note this only supports one connection at a time. With regards to L2TP I'm not sure what this implementation requires. Note protocol 47 and the GRE protocol are the same thing. I've been trying to support this for a while, with no stable solution so basically I'm using a freeswan IPSEC firewall via my internet gateways. FreeSwan is included in your SuSE distru. If you feel real brave get a 2.5.50 kernel and compile the IPSEC modules.