default admin password for G36

If you're referring to the default password as the Wi-Fi password, you can find it labeled as the Wi-Fi Security Key on a sticker at the bottom of the modem.

However, to access the GUI web portal, there is no default login password. Instead, you will be prompted to create a new password the first time you log in.

To access the GUI portal:

1. Launch a web browser.

2. In the address bar, enter 192.168.0.1 and press Enter.

3. You’ll be redirected to the Arris SURFboard setup screen.

4. Follow the Home Network Wizard, Step 1, to create your login password.

I have seen a few that the last 8 of the serial are the initial password (or password) and then it prompts you to change it. Have you tried those? 

Or where exactly are you having issues?
did you set it up with the app first?