You've reduced the speed of the interface by a factor of 10. Check the cable for damage. Check with the manufacturer of your Ethernet hardware to see if there's a driver update. Internet Gateway in Network Manager usually means either the SSDP and/or UPnP service is running. SSDP is nothing to worry about, but you might want to check the configuration of your router to ensure UPnP is disabled (it opens ports automatically, a bit of a security risk).