This topic is covered in further detail on this page.
Forgetting the password to your Gmail account can be a huge inconvenience, especially when your Gmail account is at the center of many of your online activities, as is the case with most Gmail users. However, just because it's a pain, that doesn't mean that it has to be the end of the world.
In fact, recovering your Gmail password is fairly easy and to-the-point. If you have forgotten your password and can no longer access your Gmail account, here are a few quick steps to get your account back.
Google Account Recovery
Go to Google's Account Recovery page and enter your account. Then, click 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 90 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.