If you know a source of Bianchi catalogs from 1955-1983, I'd like to know. You don't see them on the internet.
Prior to 1955 -
paramanubrio: CATALOGHI
After 1983 -
http://members.jcom.home.ne.jp/my_bianchi/catalogs.htm
Definately a second tier bike, but very sweet nonetheless. Campagnolo stopped making the Gran Sport in 1963, and was second tier then. Your bike could be prior to 1964, or the shop put on an older derailleur, which is common. Look for a date stamp on the hub lock nuts, more for fun as they aren't the most accurate.
If on ebay, I would expect $1000-$1400