It's a pretty straitfoward solution.
- Unscrew the drive from the drive bay.(The screws on the side of the drive).
2.Take out both the Power cable and the IDE cable(ribbon cable), and possibly the audio cable that plugs into your sound card.
Now slide the drive out the front, assuming it is a normal front. I cannot tell at this time because I do not know the exact model of your Gateway computer.
Now look at the back of the drive. There are some jumper settings. IF you already have a drive in your micron computer, these jumper settings will have to be changed. If not, then make sure the jumper pin is on the Master setting (sometimes marked with M or MS. If not, consult the drive's manual).
5.Again, I am not sure what model your Micron computer is, but I will assume it too has a normal front. Pop out one of the front plates (the things that cover the drive slots), and slide your drive in. Secure the drive in with screws, and plug all the cables in forward. Don't try to wiggle them in. Make sure the other end is connected to the Secondary IDE port, normally coloured(consult your motherboard manual otherwise).
6.Next you will have to determine the Jumper settings. I will leave this until I know how many drives you are planning to have in your computer.