How do I disable GMail?
As noted in the answers to the questions you linked to it isn't possible to just sign out of the Gmail application and not other Google services. 

In order to achieve the functionality you want though have you considered using an application lock such as APP Lock? With this you can require a PIN to open the Gmail app. Whilst it doesn't turn off email access it will prevent the whole family from being able to access your mail.

N.B. If you have kids using it as well then it might be worth requiring a PIN in order to purchase anything on the Play store, this might prevent some nasty surprises appearing on your credit card bill. You can see how to do this here.