The "Connect monitor to alternate video port" message suggests that it is the motherboard video port. Since your computer has a video graphics card, the motherboard connections are disabled in the BIOS. (This avoids conflicts.)
Many video cards have several display ports each of a different type (VGA, DVI, HDMI, DisplayPort (less likely on a low-end desktop)) . Some of these cards can drive two monitors at the same time. Look at where your current monitor is connected, then consider an adapter to connect the second monitor to that video card. (For example, a DVI to VGA adapter will let you connect a VGA monitor cable to the DVI port.) Some DVI ports will let you run two monitors from the one port with a splitter.
Gateway has support documents for their computers here: http://support.gateway.com/us/en/support/default.aspx
. You'll need to use the model number and look for the User's Guide, Top Answers and support documents tabs.
The only way to use the blocked port would be to enable it in the BIOS (usually press F2 on start up for Gateways). Press this key repeatedly from power on until it responds. I frequently miss the timing, just shut down and try again. However, first you would need to remove the graphics card drivers (uninstall program (Catalyst for AMD/ATI) and the drivers) from your Windows OS. Then remove the graphics card from the desktop. Since this requires opening the computer, you will need to be careful to remain grounded relative to the computer. (Use a ground strap or work carefully touching a metal surface of the case.)
Please add a comment with your computer's model number and I'll be glad to give you specifics on running two monitors from your system.
I hope this helps.