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.
What modem did you buy? When you test speeds, directly connect to the modem via ethernet ( you'll have to powercycle the modem everytime you connect a different device to it.) No router/WiFi.