Your Pi can be used as a hotspot, providing network connectivity to other devices connected to it via WiFi or USB. The hotspot plugin enables you to easily configure either one.
This example uses a hotspot served via WiFi wlan0 with traffic forwarded to the internet via eth0. The config file contents are described here.
device=wlan0
hsenable=y
hsname=myhotspot
ipforward=eth0
type=routed
wifipassword=SecretPassword
wifissid=myhotspot
Invoke the hotspot plugin with --plugin hotspot:config=/path/to/hotspot-config.txt
Similarly, your Pi can provide a hotspot via usb0 (tether host) with traffic forwarded to the internet via wlan0 or another device of your choice (e.g., eth0).
When a device such as a Pi configured for tether (client) is plugged into a USB port, the usb0 hotspot will be activated. The wifissid and wifipassword arguments aren't used with USB hotspots.
device=usb0
hsenable=y
hsname=myhotspot
ipforward=wlan0
type=routed
wifipassword=SecretPassword
wifissid=myhotspot
Change the ipforward device to be the device that is connected to the network you want to route to.
The USB hotspot plugin is configured the same way as a WiFi hotspot: --plugin hotspot:config=/path/to/usb-hotspot-config.txt
Configure a Pi with the gadgetmode plugin and plug this tether client via USB to a Pi with a USB hotspot plugin (above), or an appropriately configured MacOS or Windows system.
Invoke the gadgetmode plugin --plugin gadgetmode:static-mac
gadgetmode plugin documentation