Attached is network diagram.
In my scenario, one PC cannot ping another pc which is behind VPN at other site and has no gateway setup.
I tried lot of things with route add but none of them working.
Pls explain me why.
See diagram.
Thanx

You have answered yourself... depending on your setup proxy-arp could be a solution, but why not set an specific gw on PC3 to reach PC4 network?In my scenario, one PC cannot ping another pc which is behind VPN at other site and has no gateway setup.
For that (and good practices) you'd better set the GW then restrict internet access for PC3 at the firewall...i do not want to give internet access to those PCs so not assigning GW.
I am not familiar with proxy, but taking a step back.i do not want to give internet access to those PCs so not assigning GW.