In order to change the gateway in an Unifi device via CLI, you should first look at the current routes.

route -e
Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
default         192.168.1.1     0.0.0.0         UG        0 0          0 eth0
192.168.20.0    *               255.255.255.0   U         0 0          0 eth0

Next, we should delete the default route.

route del default

Finally, we should register the new default route as the gateway. In my case the gateway is at 192.168.20.1.

route add default gw 192.168.20.1 eth0

If we check again the curent routes, here is the result:

route -e
Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
default         192.168.20.1    0.0.0.0         UG        0 0          0 eth0
192.168.20.0    *               255.255.255.0   U         0 0          0 eth0