Your Gateway should have the power cable plugged-in in one of those ways, either on the side of the Gateway or directly to the core. It is either a barrel plug cable or a Mini USB cable. It can be plugged in like any of the below four pictures.
Step 1: To change the Gateway, start by unplugging the black power cable from the Gateway.
Step 2: Then you need to detach the white cable linking the bottom and top parts together.
Step 3: Finally, the two parts are holding together as they are screwed in one another, you can then separate them by turning the bottom part anti-clockwise.
You can now place the new Gateway on the ceiling and proceed to the same steps, in reverse order:
Screw the two parts together (bottom and top)
Plug the white cable from the bottom part into the core of the top part
Plug the black power cable into the Gateway
The lights on the Gateway should turn on