The following excerpt from an Avaya user guide is a description of the differences (it also included the 9621G).
"The letter G in a deskphone model name indicates the presence of a built-in Gigabit Ethernet
adapter that increases the transmission speed of data.
The 9621G, 9641G, and 9641GS deskphones provide a color LCD display. 9621G deskphones
have a smaller display area than 9641G deskphones, and do not have a USB interface. 9641GS
deskphones have a 5.0 inch capacitive TFT screen as compared to the 4.7 inch resistive TFT
screen on the 9641G. Thus, the 9641GS provides a higher touch sensitivity, and a brighter and
sharper look. The 9641GS deskphones provide the Ethernet link and activity LED for the network
port, located on the back panel of the phone. Unless specifically stated in this guide, the three
models are essentially the same in terms of features and functionality."