The computer is telling you your hard drives has a problem. On booting the computer has many files to read to bring the computer to run. There is a boot sequence and each file must be read in turn. It would appear this sequence is interrupted by not reading a file on your hard drive.
Although you think this would be your dvd player, it can be the hard drive disk.
Now there is another reason which may just be your problem. If you were playing a game or using a disk in your computer just before this happened, you may have ejected your dvd or cd and the computer is looking for this as it thinks you want to resume your game or program because the game lets say has modified your sequence from a normal boot to wanting to resume the program you were using. The game or program may have locked up and you could have removed the disk to reset your computer.
The first problem with our hard drive probably needs to have your start up repaired, the hard drive may need replaced soon. Search on the net using your computers model number to learn how to repair a boot problem.
The second problem could be as easy as replacing the disk you were using in the dvd player before you do the control alt delete, and you will be fine. Exit the program or game then eject the disk.
On a long shot this also could be a usb thumb drive not ejected properly, I am taking a guess you are running Winnows XP or Millennium or an earlier system as Gateway has not been a brand for a wile. The older the operating system the more this boot trouble happens, although usually one more boot can correct the fault some times.