Weird – swear I replied to this yesterday.
Any customizations made through the Customizer will never be lost, unless you delete them. Updates will never touch those.
The only customizations an update will remove are file changes to the core files of the theme.
So, if you’re wanting to add custom CSS and PHP functions, a child theme is a good idea.
You can download our blank child theme at the bottom of this page: http://www.generatepress.com/knowledgebase/child-theme-working/