Upgrade and purchase a new Modem

Try this link. Basically the same thing but without the period at the end. Not the first time a moderator has made this mistake.

As for what a gateway is, that is a modem, router and phone MTA combined into one. All the devices under the first section of that page are gateways with the exception of the TM3402 which is a eMTA(no router). The TM3402 shouldn't be bought though since Cox offers it as a free lease if you have phone service.  The other gateways are for rent. Also, it doesn't have a router so you would need to buy one of those.