Hi Luigi,
I’m not sure that the portfolio item support to include the shortcode inside the item, I will do a testing for this case.

NOTE: Imo, you can use this beauty module: https://www.joomshaper.com/joomla-extensions/sp-easyimagegallery
By using sp easyimage gallery, you can create multiple gallery with multiple image for each gallery when clicking 🙂

Example http://demo.joomshaper.com/extensions/others/column2/sp-easy-image-gallery