@mauro-tridici The path ends up being like this:
packet goes out from 192.168.120.111 to 192.168.120.1
192.168.120.1 sends the packet to 172.16.3.10
The PC at 172.16.3.10 doesn't need to use a gateway, it knows how to talk to 192.168.120.111, because it's on that network, so sends a response out from 192.168.120.10 to 192.168.120.111
192.168.120.111 is not listening for a reply from 192.168.120.10 so ignores it