Before you part with the phone, you’ll need to remove your Google account from it. After backing up the phone, go into the phone’s Settings app. In Passwords and Accounts > Accounts for, tap the account you want to remove. Select “Remove account”, then confirm. Repeat these steps until there are no more Google accounts on the device.
You can read more about removing or adding Google accounts on your Pixel phone here: https://support.google.com/pixelphone/answer/2840815?hl=en