Find Default Gateway in Linux using following options:

$ ip route show
---------------
default via 192.168.1.1 dev eth1  proto static 
192.168.1.0/24 dev eth1  proto kernel  scope link  src 192.168.1.100  metric 1

Alternative option using route:

$ route -n
----------
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         192.168.1.1     0.0.0.0         UG    0      0        0 eth1
192.168.1.0     0.0.0.0         255.255.255.0   U     1      0        0 eth1

The above output shows my default gateway is 192.168.1.1. UG stands for the network link is Up and G stands for Gateway.

Find Default Gateway in Linux using following options:

$ ip -6 route show
2001:1c00:2e16:720a::806 dev enp1s0 proto kernel metric 100 pref medium
2001:1c00:2e16:720a::/64 dev enp1s0 proto ra metric 100 pref medium
fdaa:66:67:a::806 dev enp1s0 proto kernel metric 100 pref medium
fdaa:66:67:a::/64 dev enp1s0 proto ra metric 100 pref medium
fe80::/64 dev enp1s0 proto kernel metric 1024 pref medium
default via fe80::ee08:6bff:fe84:2043 dev enp1s0 proto ra metric 100 pref medium

In case of DHCP the gateway is communicated via the DHCP options. Here is the list of the most common DHCP options exchanged with clients:

• DHCP option 1: subnet mask to be applied on the interface asking for an IP address
• DHCP option 3: default router or last resort gateway for this interface
• DHCP option 6: which DNS (Domain Name Server)

In case of manual configuration the gateway is specified in the /etc/network/interfaces file:

cat /etc/network/interfaces 
---------------------------
auto lo
iface lo inet loopback

auto eth0
iface eth0 inet static
address 192.168.1.100
netmask 255.255.255.0
network 192.168.1.0
broadcast 192.168.1.255
gateway 192.168.1.1

IPv6 works very differently from IPv4 when it comes to gateways. With IPv4, you need to specify a gateway manually or via a DHCP option. With IPv6, the IP stack locates gateways automatically by using ICMP-v6 Neighbor Discovery packets. Specifically, a client will listen for Router Advertisements, which may be sent gratuitously by a router or as a response to a Router Solicitation package. In addition to that a default route can be set with:

# ip -6 route add default via xx:xx:xx::xx dev eth0