What model Gateway is it? What OS?
You could try this;
- Leave the monitor connected to your onboard video
- Boot into your OS and install the drivers for the PCI card
- After it's installed, depending on your OS, you could use the 'Display Properties' to make the PCI card the Primary Display device.
- This should at least enable the PCI card and let you use it. I'm not sure if you'll see all the BIOS stuff during bootup, but it should "turn on" when your OS loads.