Cisco Multi-point GRE Routing

When you use tunel mode gre multipoint you have to use numbered ip on the tunnel interface (you can't use ip unnumbered loopback/fastEthernet etc..).

interface Tunnel0
 ip address 1.0.0.1 255.255.255.0
 no ip redirects
 ip nhrp map 1.0.0.2 TUNNEL-DESTINATION-ROUTABLE-IP
 ip nhrp map 1.0.0.3 TUNNEL-DESTINATION-ROUTABLE-IP
 ip nhrp map 2.0.0.4 TUNNEL-DESTINATION-ROUTABLE-IP
 ip nhrp network-id 192
 ip nhrp server-only non-caching
 #external ip used for tunnel source
 tunnel source FastEthernet0/1
 tunnel mode gre multipoint
 tunnel key 100

If tunnel ip (1.0.0.1) and remote tunnel ip (2.0.0.4) are not in the same subnet you need a route to the remote tunnel ip via tunnel0:

ip route 2.0.0.0 255.0.0.0 tunnel0

You can then have routes to that site via the remote tunnel ip:

ip route <REMOTE-SUBNET> <MASK> 2.0.0.4