Ha, Gran Turismo.
As said stands for Gran Touring, Injection.
TDi stands for Turbo Diesel Injection.
all golf's are fuel injected. been years since carburettors were used.
but the GTi has uprated parts, a turbo, better fuel injectors, and better brakes, tyres, suspension and other components to make the GTi a better handling, faster, better looking, with more toys and better all round VW Golf.
To actually answer your question, and not start a debate on GTi's, A GT is different to a non GT in that it will have some extras such as better suspension, and superficial things like armrests, to make it a 'Grand Tourer' ie more comfortable and a better drive.