WAN link going down in Sophos XGS 3100

Hello DDSM FSI,

Thank you for reaching out to the community, with the reference of Sophos Firewall Log file details > Network > DGD [Dead gateway detection] 
On the CLI, select option 5. Device Management, then option 3. Advanced Shell. Then change to the log directory using the command cd /log. 
tail -f dgd.log [during the time when gateway goes down] or use "cat' to print all the logs. Please share the logs with us here:
https://docs.sophos.com/nsg/sophos-firewall/19.0/Help/en-us/webhelp/onlinehelp/AdministratorHelp/Logs/LogFileDetails/index.html

Hello DDSM FSI,

Thank you for reaching out to the community, with the reference of Sophos Firewall Log file details > Network > DGD [Dead gateway detection] 
On the CLI, select option 5. Device Management, then option 3. Advanced Shell. Then change to the log directory using the command cd /log. 
tail -f dgd.log [during the time when gateway goes down] or use "cat' to print all the logs. Please share the logs with us here:
https://docs.sophos.com/nsg/sophos-firewall/19.0/Help/en-us/webhelp/onlinehelp/AdministratorHelp/Logs/LogFileDetails/index.html

XGS3100_RL01_SFOS 19.0.0 GA-Build317# cd /log
XGS3100_RL01_SFOS 19.0.0 GA-Build317# tail -f dgd.log
NOTICE Jul 21 11:45:51Z [6179]: do_config, Event: 1 for Entity: NKN and Entit
y_type: GW
CRITICAL Jul 21 11:45:51Z [6179]: do_config for NKN

NOTICE Jul 21 11:45:51Z [6179]: detachNode, Entities count: 2
NOTICE Jul 21 11:45:51Z [6179]: Configuration updated
NOTICE Jul 21 11:45:51Z [6179]: Executing : <gateway:gw_dead_to_live> args :
<{ "param": "@NKN", "targets": [ ] }>
NOTICE Jul 21 11:45:52Z [6179]: do_config, Entities count: 2
NOTICE Jul 21 11:45:52Z [6179]: Configuration updated
WARNING Jul 22 04:13:07Z [6179]: Actiontree, Live to Dead
NOTICE Jul 22 04:13:07Z [6179]: Executing : <gateway:gw_live_to_dead> args :
<{ "param": "@NKN", "targets": [ { "proto": "PING", "ip": "14.139.254.65", "port
": 0 }, { "proto": "PING", "ip": "8.8.8.8", "port": 0 } ] }>

Hey DDSM FSI,

So, if the gateway is still down, then On the CLI, select option 4 for the device console, and execute the following command:
console> system diagnostics utilities arp ping interface <Interface> <ipaddress>
e.g. system diagnostics utilities arp ping interface Port2 <ISP gateway> 

And see if you receive a unicast reply ? And the gateway comes up ? 

Yes i am recieving Unicast reply but gateway is still down. No change

Can you show us the config screenshot of the Network > WAN link Manager > IPv4 gateway ?