Gmail will ask you to reset your password any time they detect suspicious activity related to your account. To prevent unauthorized access to your information, please choose a new password before signing in to your account.
See this link:
Here is what Google has to say about this type of problem:
To recover access to this account, try signing in to your account through www.google.com/accounts with the last password you remember using. If your sign-in is successful, you’ll be asked to verify your phone number. Once the verification is complete, you can safely continue using your account.
If your password is rejected, or if you are directed immediately to this page after signing in, please visit our password assistance page. Once you enter your Google Account email address and type the letters in the distorted picture, you can fill out your information on our contact form. Make sure you provide accurate and complete answers.
We apologize for any inconvenience this may have caused, and thank you for your cooperation.