To be honest with ya, I would recommend you try this out on another PC, other than a Gateway.
I looked up how to disable the RAID on your pc, you will uninstall NVidia mediashield, from Windows, and then disabling RAID within BIOS, by hitting F9 on startup. Info on the subject can be found
here.
But by the sounds of your PC tech skills, it would best and safe if you did this on another machine, and not risk your own just to setup an HDD.
I was planning to run a format soon so I'll probably try that soon, also another question I had is does the type of HDD matter? It's a seagate or the normal one from the old HDD.