In my Samsung Galaxy Grand whenever I select the Gmail icon, it automatically takes me into the inbox, it doesn't ask me to give credentials like we do in laptop/desktop.
So the problem is when somebody else wants to use my mobile, they are able to see my all inbox mails etc etc as soon as they choose the Gmail icon.
So I want it set so that when I access the Gmail icon in Android, it should ask me to give username and password everytime .
The solution to your problem is using an "App Locker", which lets you protect selected apps with a password (or pattern). Each time you want to open a protected app, you then have to first "unlock" it. There might also be solutions permitting you to keep the "unlock" cached, e.g. for a confugured time span or place (e.g. when home) -- but as I don't need any app-locker I never remember which app offered that.
Examples for such app-lockers are Smart App Protector(App Lock), supporting password, pattern, and even gesture unlock, and Smart AppLock (App Protector):
Smart App Protector and Smart AppLock (Source: Google Play; click images for larger variant)
The reason for this is that the Gmail app doesn't have a login function, instead it uses the Google account that you have added to the phone. You either have to add the other person's Google account to your phone and remove it after they've used it (which is time-consuming) or tell them to use the browser to access their Gmail, which can be set to either remember or forget the user, like the desktop site.
