It cuts the Zürich-Milan journey time for passenger trains by about an hour (from 173 to 111 minutes) accelerating train trips from Germany/Switzerland to Italy significantly.
http://www.24heures.ch/suisse/Le-tunnel-de-base-du-Gothard-r...
The article says "... the journey time for travellers between Zurich and Milan will be reduced by an hour to two hours and 40 minutes."
I cross-checked that with http://www.nbcnews.com/news/world/switzerland-s-gotthard-tra... ("reducing the journey times between Zurich, Switzerland, and Milan, Italy, from just over four hours to two-and-a-half hours") and http://content.time.com/time/business/article/0,8599,2026369... ("the journey time of three hours and 40 minutes from Zurich to Milan on the Cisalpino tilting train will be cut by an hour".)
You can see these all give a time of over 150 minutes using the tunnel, not 111 minutes. They all agree it's a savings of about an hour.
Edit: we will be looking at journey times of about 170 minutes from Zurich to Milan AFTER the tunnel opens.