Just had gigabit installed on June 1. Speed was ~400 Mb/s down and 40 Mb/s up. Since then it averages 250-350 down and a constant 40 up. The speed is the same as my old Blast tier. I have the Gigabit Internet tier and the Technicolor CGM4140COM Gateway.
First, make sure you are actually on the gig speed tier. Visit https://mydeviceinfo.xfinity.com to determine your speed tier. After you log into your account, your speed tier will be displayed on the left-hand side of your screen. Alternatively, you can view the speed tier to which you are subscribed here. Sign in and scroll down to the data usage graph area.
Other things to try:
I was and he connected to the box with his phone and used a WiFi signal. The box plugged into the gateway via Ethernet cable. Unless the iphone has a mysterious way to connect directly to the black box, it used a WiFi signal. He did not use a computer at all.
I don't know if you weren't paying attention or not but that black box, plugs directly into the gateway, it's not wireless.