Gmail – How to log out of Gmail of all devices from desktop

At the bottom of your inbox in Gmail there is something that says "Last account activity:"
Directly under that there is a link "Details"
If you click this link it should pop open a new window with the activity log.
At the bottom of the log it will give you the option you are looking for.
"Alert preference: Show an alert for unusual activity. Change"

If you request that an alert be generated,
it is my understanding that "strange" access attempts may be prevented.

Obviously, there are safety implications to disabling such an alert.

No, it won’t, unless you specifically do this.

Forgot to sign out?

If you've forgotten to sign out of Gmail on another computer, you can sign out of your other sessions by going to the bottom right corner of Gmail, clicking Details, and then Sign out all other sessions.

Source.

Also, note that when you sign out from your tablet (or phone for that matter), you sign out of all Google apps you might have on your device for that account.

Source.