The full width additional images setting is located in Product Settings > Catalog Design > Product Page. Check the checkbox to enable it.
After enabling this settings the addtiona images will be displayed in full width even if product gallery width is not 100%. Additional images will appear under the image, short description, price and shipping options but above product attributes.
When product gallery width is set to full width the layout will change even further. The main image will show up on top in full width, than the short description, price (if enabled) and shipping options (if enabled). Under it the additional images will appear.