push the menu button on the monitor, scroll through the monitor menus, there should be a horizontal and vertical size adjustment.
You should be able to adjust that so that the image does not spill off the sides of the viewable area.
Then there should also be a save option, to save the settings.
On some older monitors, the settings are lost when the monitor looses power.
Also go to the gateway website and download the manual for the monitor to familiarize yourself with the setup adjustments.