Ctrl+Scroll is a shortcut to increase or decrease the viewport size in many applications such as your web browser, Libreoffice, Nautilus, Evolution and your preferred image viewer. Gedit should allow this behavior as well. It is in particular useful when you are performing a talk or a presentation and need to show code to an audience in a large room. 

For example when Jakub Steiner need to show the contents of his XML files when he demonstrates how GNOME Design creates Localized Animation using Blender (at 8 minute mark) at Libre Graphics Meeting: 
http://medialab-prado.es/mmedia/10801/view

The potential 25 seconds it took to enlarge the text using the current behavior could have been decreased to less than a second with Ctrl+Scroll. Support for Ctrl+ and Ctrl- could be added in the case where you are using a touchpad - this is also two very common shortcuts used for the same behavior by the same applications stated above.