...
There are two parameters passed to the uninstall() method. The first ($gateway_id) is the ID of the gateway being uninstalled. Because a gateway can be installed independently under different companies you may need to perform uninstall logic for a single plugin gateway instance. The second parameter ($last_instance) identifies whether or not this is the last instance of this type of gateway in the system. If true, be sure to remove any remaining remnants of the gateway.
...