You must follow Google’s account recovery instructions in order to retrieve your Gmail account using a Facebook account. In order to confirm your identity, you typically need to visit the Google Account Recovery page, input your Gmail address, and then follow the instructions.

You must link your Facebook and Gmail accounts in order to utilize Facebook to log in to your Gmail account. To achieve it, follow these steps: Click the “Email or Phone Number” button in the upper right corner of Facebook. After entering your Gmail address, click Proceed.

Recover Gmail Using Facebook

How do I recover my Gmail account from Facebook?