I can't say for sure, but in the Carrozzeria Viotti book it only gives the 1100/1200 the name 'Gran Turismo'.
Gran Luce only means 'Big Light' literally of course and according to Alessandro Sannia, writer of the book Carrozzeria Viotti, "...a four-seater version of the (600) coupé was developed... called the 600 Granluce, a name that identified (also for Fiat) cars with a very large glass area."
Note the spelling of Granluce as one word, which signifies the name of the car, as opposed to two words Gran Luce which means large glass area. It could be that whilst the name of the car is Gran Turismo it also carries the description Gran Luce.
I'm just guessing, but as far as I can see the car in your puzzle is called Gran Turismo but not actually anything else as a name, though it might be called Gran Luce as well because it was part of the series of cars which had panoramic windows, which were just starting to be popular around the time of the car's appearance in 1958.