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 9 months 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.