Started with a fresh clean install of Gargoyle 1.13.x on the Netgear WNDR3700v4 router. This software image already includes USB basic support.
SSH into the router and installed tethering packages by issuing these commands at the command line prompt:
$ opkg update
$ opkg install kmod-usb-net kmod-usb-net-rndis kmod-usb-net-cdc-ether usbutils udev
Some of the packages are already installed, so the installer will just provide a warning message and skip them.
Connect the phone to the router via USB (I used port #2) and enabled USB tethering on your phone.
Modified network configuration file (/etc/config/network). By default this file maps your WAN logical interface to your physical WAN ethernet port. You need to map the WAN interface to the USB0 interface. At the CLI prompt entered this:
uci del network.wan
uci set network.wan=interface
uci set network.wan.ifname=usb0
uci set network.wan.proto=dhcp
uci commit network
ifup wan
After doing this, my router got an IP address for its WAN from my android phone and my LAN devices are surfing the internet flawlessly.