In plain terms GT would give you much better performance than GS. See any of the reviews of GT cards which compare them to GS and you would notice.
Not only they run at higher clock speeds, but it mostly has higher everything (all the other jargon that you hear about these cards e.g. clock speeds, memory speeds, pixel shaders, etc)
Somebody more techie could give you more technical differences, but the bottom line is, your NFS cars and environment is going to look more realistic with a GT card as compared to a GS one !