Gateway wifi speed really bad

Hi,

I pay for 500 Mbps and get close to that with a direct connect to the X1 Gateway (363Mbps).  When I switch to wifi using the Gateway, it drops tremendously.  Even when I'm sitting right next to the Modem I only get 77 Mbps.  In the next room, it drops to 45 Mbps.  Upstairs I can't even get a connection.  I bought and added in the XFi pods, and now upstairs I get 18 Mbps.  I've reset the Gateway numerous times.  What is the max wifi throughput of these Gateways?  How can I improve my speeds?  The gateway is on a desk, not next to anything metallic, a foot or so away from a wall.

Thanks,

Mike