I have never tried this but it should be possible. You can configure a GRE tunnel as described in the KB but without IPSec. As there are no VPN SAs the tunnel interface will not come up. But routing through the tunnel interface may be forced if the routes are configured as permanent. The IPs of the VPN endpoints should be routed across the GRE tunnel.
As I suppose you want to terminate the VPN IPSec on a third party device(s). If both devices are SSGs this does not make sense to use IPSec over GRE.
