Users must agree to a disclaimer before they are allowed to add items.
In the upload modal window a user is presented with a checkbox to signify they agree to the terms and conditions of the site.
This module may be downloaded from the Gallery Codex here.
To install, extract the "upload disclaimer" folder from the zip file into your Gallery 3 modules folder.
Afterwards log into your Gallery web site as an administrator and activate the module in the Admin -> Modules menu.
The [Gallery3:Modules:Pages] module is highly recommended to create a terms and conditions page.
Once that mdule is installed create a page called 'terms_and_conditions'. Add your content to the page as desired.