if you already have the backup disks that came with your gateway... put disk 1 in the computer and turn it on it should start the restore prosses.... you may have to adjust your BIOS to boot from DVD first.
Did you create the backup discs?Do they include your personal data? If yes, try to follow steps from anonymous1.
Computers do not come with Restore disks anymore. The recovery information is stored in the hard disk in a hidden partition (about 12GB) and people are supposed to create the discs right after buying a computer. A practice that nobody does.
So you need to put the original hard drive back into the computer. Turn the computer on and hit F11 (I think) to bring up the recovery screen. There might be an option to create the restore discs. Another way is go in Windows/Start and there is a folder Gateway (or Acer) Management where inside there is a utility for you to create the discs. Once the discs are created (maybe 4 DVDRs), you can use them on the new hard drive to restore the OS and apps in it.
If the original hard is completely inaccessible, you need to do a generic Windows install and install all the drivers (downloaded from Gateway website). At the bottom of the laptop there is a Windows sticker that shows the Windows version (seems to be XP) and the key code. Find, buy, borrow a Windows OS disc that matches with Windows version on the sticker and install it (using the keycode on the label). After installation you need to download all the drivers.
Remember...all you data is in the original hard drive.