If you're signed in to your mobile device with your Google Account, you're signed in automatically to the Gmail app. If you're using Gmail in a browser, you might be signed in automatically if your browser has saved your username or password.
Stop saving my username or password
If you don't want to be signed into Gmail automatically, change your browser's settings or preferences so that your passwords aren't saved. If you don't use Google Chrome, search your browser's Help Centre for information on how to stop saving passwords.
Temporarily remove your username and password
To temporarily remove your username and password from the page, clear your browser's cache. If you don't use Google Chrome, search your browser's Help Centre for information on clearing the cache.