[Plugin: NextGEN Gallery] Update deletes gallery template

That’s why you should copy the template-files to your theme’s folder and never edit files inside the plugin itself (the first method – within the theme – is update-proof).

Do you mean that if I put the template file inside the theme folder it will be accessed by the plugin? I didn’t find a similar suggestion in the “documentation”.

See https://nextgen-gallery.com/templates/