I have a garage, and want to know what car to buy. haha
You need to give WAY more info. What model gateway is it? lets start with that. If you can give info on the power supply that is in it, that's great to. Does it have an open AGP or PCI-E slot on the motherboard? What size, or resolution, monitors will you be using? What will you be using these screen for? What is your budget?
you may not need to buy another graphics card. any graphics card,for as far back as i can remember, can support two monitors. telling us more about what you will be using the monitors for will be helpfull.