You are correct. Conceptually, I dont see a problem. As far as the GRE Source and Destination IP Addresses are part of the interesting traffic between the VPN Servers, the IPSEC Tunnel as well as the GRE Tunnel should work fine.
Below is an URL that discusses a similar scenario with the Pix Firewall.
Login to the FXOS chassis manager.
Direct your browser to https://hostname/, and log-in using the user-name and password.
Go to Help > About and check the current version:
Check the current version availa...
We have configured the outside and inside Interface with official ipv6 adresses, set a default route on outside Interface to our router, we also have definied a rule , which also gets hits, to permit tcp from inside Interface to any6.
In Syslog I also se...