The problem being.
If it will fit into the Gateway box. And then if the power supply would recognize it. I believe the HDs are also configured to work only with Gateway MOBOs, too.
One could put the components into a new box and MOBO and still have the issues of power supply fitting and interacting with the new MOBO as well as the HDs.
All that so Gateway can sell their own MOBO at a price nearly equalling a new computer.
At least that was my experience with them.
Arf