RouterA
Public IP: 217.219.50.100
Tunnel IP: 192.168.0.1
Private network: 192.168.1.0/24
IP on that private network: 192.168.1.1

RouterB
Public IP: 217.219.97.1
Tunnel IP: 192.168.0.2
Private network: 192.168.2.0/24
IP on that private network: 192.168.2.1


Config on RouterA :
interface Tunnel0
description connection to RouterB
ip address 192.168.0.1 255.255.255.0
tunnel source Serial 1/0
tunnel destination 217.219.97.1
tunnel mode gre ip

ip route 192.168.2.0 255.255.255.0 Tunnel0


Config on RouterB :
interface Tunnel0
description connection to Linux RouterA
ip address 192.168.0.2 255.255.255.0
tunnel source Serial 1/0
tunnel destination 217.219.50.100
tunnel mode gre ip

ip route 192.168.1.0 255.255.255.0 Tunnel0

Thanks to Author, orignal link http://www.en.persianadmins.com/article/cisco/Configure_Cisco_GRE_tunnel_2007080417.html