Using the following command you can find the default gateway and the use the address with ping command to ping your default gateway.
C:\Users\priyj_kumar>ipconfig /all | find "Default Gateway"
Default Gateway . . . . . . . . . : 192.168.1.1
C:\Users\priyj_kumar>ping 192.168.1.1
Pinging 192.168.1.1 with 32 bytes of data:
Reply from 192.168.1.1: bytes=32 time=4ms TTL=64
Reply from 192.168.1.1: bytes=32 time=3ms TTL=64
Reply from 192.168.1.1: bytes=32 time=2ms TTL=64
Reply from 192.168.1.1: bytes=32 time=2ms TTL=64
Ping statistics for 192.168.1.1:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 2ms, Maximum = 4ms, Average = 2ms
This way you can find and ping your default gateway.