Hi there and welcome
To do this you need to first click on your name in the top right in Gmail and click Sign out.
then on the sign in page, uncheck the box "Stay Signed in" and then sign in
When you do this, each time you close ALL browser windows, then you will have to sign into Gmail again.
If you have any problems, or would like any additional information or assistance, please do not hesitate to let me know.
Else if you can take a second to rate my service by clicking one of the stars at the top of the screen then submit, that would be greatly appreciated.