On a default G2 install, everything is visble in G2. Zencart does have the option of requiring logon to view, and there are various ways of restricting products viewed by user.
However, a proposed model for wedding photos and the like was to point the potential customers to the gallery, with a logon for that gallery, allowing them to use the 'purchase options' button in Gallery to switch to Zen Cart. Would probably need some slight mods to the Zen Cart templates, as the 'Back to Gallery' button is currently only visible on the product pages.
This could be added to the shopping cart view, or make add to cart return to the product page for instance.