Well, assuming the 2nd monitor was good, and used a separate data cable, sounds like a video problem with the computer. Does the computer have a video card or does it use onboard video?
In either case, you'll need to add a separate video card.
If you need help determining what type of video card to get, post back with your exact Gateway model number.
EDIT: I knew this wasn't right when I wrote it
If you have onboard video but have been using a discrete video card, then remove the video card and try the onboard video.