A gateway is a single device that can do the jobs of both a router and a modem. If you own a dual router and modem device, then it will connect your ISP’s network to your local WiFi network or Ethernet. This is often seen combined with a phone service in the same hardware device.
In the broadband industry, gateway is often colloquially used to refer to a device that is acting as a primary node (forwarding host), connecting other access points or networks. In this case, gateway may be interchangeable with a default gateway or primary router.