From my current location.
Kernel IP routing table
Destination Gateway Genmask Flags MSS Window irtt Iface
default c-71-203-6-1.hs 0.0.0.0 UG 0 0 0 eth0
18.104.22.168 * 255.255.254.0 U 0 0 0 eth0
loopback * 255.0.0.0 U 0 0 0 lo
The second line, 'default, next column is the current gateway.
How to know what should be there depends on where you are connected.
When I took that info, I was directly connected to the modem supplied by Comcast, so the gateway addr was set when my system did a DHCP request for an IP address. I got an IP, and the Gateway plus the DNS server addresses.
If I were connected at home, the IP address for gateway is set in mt router config. I got the IP address from the ISP. All the ones I have dealt with provide that if you talk to them, or look on their web site. At home, the gateway is set to the IP address of the router connection on the lan side ( not the ISP side ).
Does that help?