In addition, this post might help, as it also gives a short rundown of what you might need for internet service equipment -
You have a modem, and then a router. Some boxes are combo boxes (include both) The modem needs to be Docsis 3.0 or higher, (Docsis 3.1 for the 400Mbps and up speeds) You will also need a phone port if you have/want phone service.
You need a DOCSIS 3.1 modem for Gigabit, but for Extreme Pro (400 Mbps), a 24x8 channel DOCSIS 3.0 modem like the Netgear CM600 will work just fine.
Comcast does rent their XB6 Advanced Gateway for both Extreme Pro and Gigabit, but only because their XB3 gateway has only 16 download channels.