Remember to add the ip_gre module first in the linux box prior to configuration.  You can load the linux commands in /etc/rc.d/rc.local file to make sure the module & configs are run.. much like the autoexec.bat in ms-dos olden days.

reference 1:

reference 2:

CISCO to LINUX GRE TUNNEL

CISCO ROUTER CONFIG
!
interface FastEthernet4
description Interface connected to Internet
ip address 10.10.10.1 255.255.255.252
!
interface Tunnel1
description connection to Linux Network
ip address 1.1.1.1 255.255.255.0
tunnel source 10.10.10.1
tunnel destination 20.20.20.1
tunnel mode gre ip
!
! — network behind the linux router —
ip route 192.168.200.0 255.255.255.0 Tunnel1
!

LINUX ROUTER CONFIG

# modprobe ip_gre
# ip tunnel add toCiscorouter mode gre remote 10.10.10.1 local 20.20.20.1 ttl 255
# ip link set toCiscorouter up
# ip addr add 1.1.1.2 dev toCiscorouter
# ip route add 1.1.1.0/24 dev toCiscorouter
#
# — network behind the cisco router —
# ip route add 192.168.500.0/24 dev toCiscorouter

LINUX to LINUX GRE TUNNEL
LINUX ROUTER-A CONFIG

# modprobe ip_gre
# ip tunnel add routerb mode gre remote 10.10.10.1 local 20.20.20.1 ttl 255
# ip link set routerb up
# ip addr add 1.1.1.1 dev routerb
# ip route add 1.1.1.0/24 dev routerb
# ip route add 192.168.200.0/24 dev routerb

LINUX ROUTER-B CONFIG

# modprobe ip_gre
# ip tunnel add routera mode gre remote 20.20.20.1 local 10.10.10.1 ttl 255
# ip link set routera up
# ip addr add 1.1.1.2 dev routera
# ip route add 1.1.1.0/24 dev routera
# ip route add 192.168.500.0/24 dev routera