GRE is an encapsulation protocol, much like ESP in IPSEC, just without the security. GRE is commonly used in Microsoft PPTP VPN implementations(which is not just RRAS, most appliances/devices implement it to, i.e. Cisco routers), which uses MPPE for encryption. I have not tested Ettercap with GRE traffic, however, I am assuming that it removes the encapsulation, but you would still need to deal with the encryption. A good start would be Muts's article on attacking Cisco routers using SNMP and GRE, http://www.securityfocus.com/infocus/1847.