If you have forgotten your password and can no longer access your Gmail account, here are a few quick steps to recover your account.
Google Account Recovery
Go to Google's Account Recovery page and select I don't know my password. Then enter your Gmail email address and click on Continue:
If you've previously linked a phone number to your account, you can use it to retrieve a verification code by SMS, or you can click on Verify your identity and answer to the security questions.
N.B. If you have not accessed your account for 30 days or more, Gmail may have considered your account as abandoned and may have deleted it from the server. In this case, account recovery is no longer possible.