You cannot terminate GRE onto a pix, but as the last reply said you can pass GRE tunnels through a PIX, which is a far as I am aware is the only way to pass OSPF/EIGRP routing information and non IP traffic through an IPSec VPN terminated on a PIX as IPSec does not support non IP traffic or the multicast hellos of these routing protocols. So I think in some circumstances GRE through the PIX is useful.
ps For some reason GRE information has become more difficult to find since the website has changed
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...