It seems to me that the GOU must be divisable conceptually into two parts ("All Gaul is divided into three parts"? :-) )
Secondly, to take it out of order, is the mechanism to output the frame buffer's contents as HDMI protocol packets and to repeatedly do so at frame rate.
Firstly, to massage incoming MPEG messages and fill the frame buffer which presumably uses the mathematical operations of the gPU.
Where does the simple act of plotting pixels fit into this? Is this done simply by the ARM software writing directly into the frame buffer?