This is definitely possible, although I don't consider it to be a good idea.
The voltage and current draw from the gateway can easily be accommodated by the Molex hard drive power plug. You will need to get a plug for the molex connector and use a plug that fits your gateway (make sure the power and grounds are correct). Then connect the wires up. The 5V is red and the GND is black. Normally, I would solder the connections and use electrical tape to cover up anything conductive.
The issues I can think of doing this are:
- Putting the box inside the chassis may affect the thermal profile of the system and cause it to run hot or overheat.
- Something loose in the chassis may also cause damage to components.
- EMC may be a problem because the gateway was designed to be in a case. Removing it from the case may let the radiated noise affect the system's functionality or vice versa.
- You'll likely have wires for the gateway running into and out of the chassis.
- If the gateway hangs for whatever reason, it'll be a pain to unplug and power back up.
- You can't see the gateway status lights unless your chassis has a clear opening.
- Adding a box that connects to other lines in the house such as the telephone can introduce another way that lightning or a power surge could enter your system potentially destroying your computer and your gateway.