Routing fail over to another path over GRE tunnel

[jammcla]

Right now we have our offices connected with a tunnel IPSEC VPN all back to our main office.  If our main office loses internet all our other offices cannot talk to each other.

Using 3 test boxes to start I created IPSEC transport VPN with a GRE tunnel over them.  If I create static routes I can get them talk to to each other.  So instead of creating static routes on a GRE tunnel, how do I get it to a fail over configuration of a direct path from remote office to remote office if the main office goes offline?