Hello Bobster47 and welcome,
If you do not use a Comcast Static IP 1,5, or 13 block, then you can depress the little black reset button on the back of the Comcast Gateway (CG) for 15 seconds minimum, then your device will be set to factory default including your CG login password credential.
Hope this helps you out.
Then you must call 800-391-3000 and use the technical high speed internet option and request the technical agent reset your Comcast Gateway password. You can also private message any of the forum administrators like Comcast_Michael, etc. to get this accomplished.