What does gti stand for?

I believe GTI stands for "Gran Tourismo injection"

GTI stands for "grand tourer--injected" or "grand touring--injected", a designation typically reserved for sporty coupes. Although initially a technical classification, current carmakers use the acronym as a performance-oriented marketing tool.

The first car to be marketed with the abbreviation "GTI" was Maserati's 1961 3500 model. This car introduced fuel injection to the grand tourer class, hence the addition of an "I". Since then, GTI has become associated with performance.

GTI belongs to a whole group of grand touring classes, including GTO, GTR and GTA. These cars all claim high performance, comfort and luxury as defining traits, hence "grand touring"--they're supposed to be ideal for long-distance trips.

"Grand touring", or "grand tourer", are the English versions of "gran turismo"--the original Italian term for this type of car