Gateway Password

Are you referring to your WiFi home network's password, or the password for gaining access to the device's user interface pages ?

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@JulianR1 wrote:

 -If you are referring to the web interface (10.0.0.1), that cannot be reset, not by you the consumer, or by comcast.

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Sure it can. The customer can hard reset the device to factory defaults by pressing and holding in the recessed reset button on the rear for 30 seconds. The user name and the password will now become the defaults of admin and password. You will lose any customized settings, and they will need to be re-configured from scratch.

Xfinity is unable to reset it for you, nor are support reps. I know this from experience. The best you can do is:
 -If you are referring to the web interface (10.0.0.1), that cannot be reset, not by you the consumer, or by comcast. 
-If you are referrring to the Wireless Network Passphrase, you can reset/change the passphrase using the Xfinity xFi App, or the Xfinity My Account app.  You can also reset it by using the web version of xFi or My Account.

My Account Web version: https://customer.xfinity.com/#/
Xfinity xFi Web:Xfinity xFi Web Version 

Xfinity xFi Apps:

Android:
https://play.google.com/store/apps/details?id=com.xfinity.digitalhome

Apple iOS/App Store:

https://itunes.apple.com/us/app/appName/id1178765645?mt=8

My Account Apps:
Android:
https://play.google.com/store/apps/details?id=com.comcast.cvs.android

Apple iOS/App Store

https://apps.apple.com/us/app/xfinity-my-account/id776010987