If I am reading your post right, you have tried connecting the PC to the gateway device via an ethernet cable.  Are you plugging into the yellow ethernet ports or to the red one marked "broadband"?  You should be connected to the yellow ethernet ports.  Plugging anything into that red port could result in the gateway disconnecting itself from the network due to input confusion ( in case of a reset or restart).  The red port is only for FTTP installations.  If your plugged into the correct port then the only other thing I can think is that your PC's NIC is either disabled or malfunctioning.  Open your network connection manager and check to see if your onboard NIC is enabled or not.  If everything is plugged in the right way and your PC is configured the right way then I would recommend you restart the gateway. Go around the house and unplug all of your settop boxes and then unplug the gateway device.  Wait about 20 seconds or so and then plug the gateway back in.  After several minutes it should sync to the network (broadband and service lights both solid green).  Then go around and plug all of the settop boxes back in and let them load up one at a time.  If your still not getting an internet connection out of the yellow ethernet ports it may be time to call technical support.  They will undoubtedly have a regimen of troubleshooting steps for you to follow. If that doesnt work, they will send someone out to your house to take a look at it.