When you log in to Gmail through the browser (not the push mail) on Samsung Galaxy Tablet, and which you use is the default browser (Samsung default internet browser tab), there is no menu to logout or exit from Gmail.
To be able to logout / exit from Gmail that uses Samsung Tab Android default browser, or the way the trick is to go to theBasic HTML version
Can be seen in the following example display my gmail below
Click on Basic HTML, so the page will display the basic html version of Gmail, which has no option for Logout / Sign Out
Click On Sign out to get out of your Gmail.
May be useful.