Look up the ethernet driver from the gateway website, use your computer model number to find the right driver, download the driver to your desktop from Gateway.
turn off the anti virus and firewall, install the driver, restart the computer and turn the anti virus and firewall back on.
With the computer on, unplug the power from your (cable, DSL?) modem, wait 15 seconds, plug the power back in to the modem, wait till all the modem lights are all flashing.
Then restart the computer.
This should usually get you back into the internet connection.