If you have accessed your Gmail account from a friends computer, or a Internet Center, and have forgot to logout from there, then your account can be used by anyone who has access to that PC, which is when you should lot out from that PC asap and for this Google has offered a very good option called as Remote Signout.
Once you login to your Gmail Account, in the footer section you will find a small link giving you details on your current IP location and the last known activity. Click on the details link and you will be shown another window with a detailed report of your last 5 sessions which show you the mode on how the connection was made along with the ip address. Now you will have to click on the button which says – Sign out all other sessions, and your account is now safe. Check out the screenshot below to get a better idea.