You cannot logoff of Gmail on your mobile
phone ever!, But you can add a password,pattern or whatever you like best. Also the Google Play Store you can find many good apps to lock the O/S completely or just Gmail.

If you are wondering why Google made it work
 this way? Is for ease of use mainly and there is a post
on this site as Paul from Google told me why it works
that way and Google has no plans of changing this policy now or in the near future.

Good Luck