I don't believe that the firmware loads in the Comcast supplied gateway devices support that feature. You would need to put a dynamic DNS client on one of your computers, or use your own router that supports the dynamic DNS service of your choice.
Would you happen to know how often on average Comcast changes our IP addresses? My router stays on all the time so I'm assuming it's fairly stable at one IP address?
A residential connection is assigned a dynamic public IP. Although "dynamic" means that it can change, the Comcast assignments are known as being sticky dynamic and are tied to the MAC address of the device that is connected to the LAN interface of the cable modem. In simpler terms, the IP address doesn't change very often normally. At one point in the past, I had the same public IP for over two years. And this is even after going on vacation for a month and having the modem unplugged. When I returned and powered things back up, I still had the same IP.