Nvidia has used different prefixes over the years, indicating more and more powerful CG:
GS / GT / GTS / GTX
the prefixes have reduced nowadays, in 2014 there are only GT and GTX, GT designating the entry level and GTX the medium and high end, but the border is not very clear
the reason is essentially commercial, sometimes the same GPU has the 2 names depending on the memory or the frequencies, it does not therefore not from another architecture
For example for the GT 640, equipped with GDDR3 memory, Nvidia called GTX 650 the same GPU (384 calculation units), but equipped with GDDR5 memory:
This is all the more absurd since there is an OEM GT 640 GDDR5 (intended for integrators)
Same controversy in 2011 with the GTX 550 Ti, whose GPU had the same number of processing units as the GTS 450, with essentially higher frequencies:
https://difference-between.net/difference-between-nvidia-gt-and-gtx/
this GTX 550 Ti should therefore have been called GTS 450 Ti or GTX 450...