Thats it really. In the UK, the GT pack was an optional extra from the dealers, and most dealers pushed this "pack" like mad. Most buyers went for it as it represented good value for what you got. As Lexx says, you got the cruise control, headed electric leather seats and a bose stereo system. I forget exactly what the GT pack cost, I seem to recall it was something like £1600 extra, which on a £32k (depending on the year you went for) car wasnt much. So you`ll find most 350z`s for sale do come with the GT pack.
On top of the GT pack, you could fork out even more (£600 extra) and swap the black heated electric leather seats for Alezan Orange or Frost Grey/Blue - or at least you could in 2007 for the zed I got.
Non UK cars came with all sorts of different weird and wonderful specs, so you may find imported cars labeled as GT with slightly different specs.
I'd heard the GT pack was an extra £2.5K on the 2007 model.