I'm not quite following your issue...
What app are you using? the Gmail app or the "Mail" app. Or perhaps you are using the browser?.
Only in the case of the browser you can Logoff. In the others the only way to eliminate your data is by deleting the account. And it it's in the Mail app then the only way is to factory reset the unit.
Gmail is assumed to be your default account and most services like the Play Store run in tandem with it, so multiple functions in the phone are related to the Gmail account.
In this phones the email account is not always connected. Im saying that in case your questions is related to the gmail app constantly connecting to your GPRS service. To prevent his just go into settings in the gmail account and set the refresh frequency to "never" this will set the updates to manual.
Have a good day.
Francis.
