If GRE tunnels are to go through an IPv4 firewall this firewall has to be opened for IP protocol type 47 for IPv4 datagrams coming from or going to the remote tunnel end-point.

GRE tunnel end-points are authenticated by a simple key that is transmitted during the setup of the tunnel. Since the key is transmitted in clear text format this doesn’t really add much security and the key is also not used for encryption of any kind.