I have already loggged in to the xfinity gateway and I did use the default password for admin, but I forgot the password I created

There's no password recovery for the local gateway login, but you can reset to factory default, and the username/password will be admin/password again.

Older gateways had a toothpick hole for a reset button.  Newer gateways just have the WPS button.  Press and hold either one for 20-30 seconds until the lights start flashing and it reboots.

Hey @user_538cb2, thank you for reaching out to Xfinity Support on our Forums. @flatlander3 information is correct for self service options and a factory reset would be required to reset the Admin tool password. On some models though, us as agents have the ability to reset it within our version of the admin tool. If you can send us a direct message, I can certainly check to see if we can with yours. 

To send a direct message:

Click "Sign In" if necessary

• Click the "Direct messaging" icon near the top right of the page

• Click the "New message" (pencil and paper) icon

• Type "Xfinity Support" in the "To:" line and select "Xfinity Support" from the drop-down list which appears. The "Xfinity Support" graphic replaces the "To:" line

• Type your message in the text area near the bottom of the window

• Press Enter to send it