We usually refer to the gateway in RPL as the border router. Here is an example from the Autonomous Networks Research Group of the University of Southern California on how to simulate a border router with Cooja.
Border routers are routers that can be found at the edge of a network. Their function is to connect one network to another.
The page also links to this guide, which explains the setup of a border router:
The RPL border router is used in order to interface a regular IP network
with an RPL 6LoWPAN network. This is similar to the bridge that was previously
used, except that it also runs an RPL network. The border router is
located in the
must compile it using the following commands:
cd contiki -2.6/ examples/ipv6/rpl-border-router
(the directory referred to is available here, on GitHub)
They also specify how to select mote types in section 3.4, including the border router.