Is it possible to disable gmail sync completely, without logging out?
Disabling sync and removing email history is possible, but it will not stop others from simply enabling sync.

Option 1.
To disable sync for Gmail, go to settings -> accounts -> Google. And deselect the Gmail sync option.

Now, go to settings -> apps -> Gmail -> storage -> clear data. This way, when you open the Gmail app, it will not show the email history.

Option 2.
disable the Gmail app. Go to Settings -> apps -> Gmail -> disable. This way the Gmail app will simply not show on your home screen, and you will not get any notifications from the app.

But to fully disable sync without anyone having a way to enable it again, is not possible as far as I know.

Email could be access in many ways, like open the chrome browser and going to gmail.com there, it will not even ask for you password because the phone is already logged in.

In short your options are: lock the phone with a simple code that can be used even by seniors, or remove the current account and add a different one, or use the options above, leaving the phone vulnerable.