Since version 2.7.0 Phoca Gallery can display "Add to cart" buttons and other information from Simple Caddy e-commerce solution in Category View and Detail Window, see:
Phoca Gallery and Simple Caddy documentation site, Phoca Gallery and Simple Caddy integration demo site ( https://www.phoca.cz/stockphoto/ ) and Simple Caddy site: https://atlanticintelligence.net/