if you have 1 gmail id then you have an infinite amount of gmail ids and there is a good chance that you do not know about all these different email ids. If you email address is abc@gmail.com then try sending a mail to abc@googlemail.com. Send another mail to abc+test@gmail.com. Now check your mail box. You will find both the mails in your mailbox. Surpised?? Well I know magic 
Good TryYour gmail addresses that you do not know!!!137|just some value|7