You will be asked to enter the last password you remember. However, if you do not remember any of the previous passwords you can click on the option ask another question. Gmail, then sends a prompt to your registered mobile number and then you will have to select the option yes. A set-up numbers will also reflect on your mobile screen and you will have to choose the number which is shown on your login page of Gmail. Once you have done this successfully, you can go ahead and change your Gmail password and then access your account as before.