How to make Gmail in Android ask for user name and password as in laptop?
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)