IPSEC over GRE VPN with cisco router

[newNetwork]

Is it possible to configure IPSEC over GRE with Fortigate as a hub and cisco routers as spokes . there are two links on each site , bgp is configured on ISP interfaces with primary and backup configuration. VPN will be formed between loopback at FG and loopback at the spoke sites. those loopbacks are advertised through bgp. Is this config possible. My understanding of config steps are as follows. 1)configure bgp on each end with loopback advetised to the isp in primary and backup link style. 2)configure ipsec over GRE with external interface to peer with as loopback .this will re-route the traffic through secondary link in case the primary link goes down without breaking the tunnel as tunnel endpoints are up .