GRE tunnel

Hi Mohamed-Salah,

When you get "ICMP time exceeded in-transit" means that TTL of the packet reached zero so probably due to a routing loop. 
Can you get a routing table on both sides of the GRE tunnel and paste them here?

thank you @DPadula 

I make sure of this and the routing is going correctly from both sides as shown below

fortigate 

Routing table for VRF=0
Routing entry for 192.168.51.0/24
Known via "static", distance 10, metric 0, best
* 172.25.52.2, via xxx-store  <--- gre tunnel 172.25.52.1

Cisco 

Routing entry for 192.168.50.0/24
Known via "static", distance 1, metric 0 (connected)
Routing Descriptor Blocks:
* directly connected, via Tunnel51  <--- gre tunnel 172.25.52.2
Route metric is 0, traffic share count is 1

Hi Mohamed,

I can see that you have a route on the local device that you are using to ping 192.168.51.200.

But you haven't pasted the routing table on the remote device. Can you confirm that it has the route to 172.25.52.x?

Also, run the following commands during the ping test (remote device). 

diagnose debug reset
diagnose debug disable
diagnose debug enable
diagnose debug console timestamp enable
diagnose debug flow filter clear
diagnose debug flow filter proto 1
diagnose debug flow filter addr 192.168.51.200
diagnose debug flow show iprope enable
diagnose debug flow show function-name enable
diagnose debug flow trace start 500
diagnose debug enable